Time Capsule - older model

You can use the Time Capsule still as a device on your home network, without using it's wi-fi ability directly, by plugging it directly into your new router/modem and one of its ethernet ports. I'm assuming it has at least four ports available? This way, your old and new iMac would connect wirelessly to the new router/modem but still have the ability to use the Time Capsule plugged into the router. You would also want to turn off wi-fi in your Time Capsule altogether (using the Airport Utility app) so it doesn't create interference for your new wi-fi network.

Your existing backups will be unaffected. Turning off the Time Capsule's wi-fi and plugging it in to the router/modem via Ethernet won't change anything else in the Time Capsule itself.

This appears to be a pretty straight-forward tutorial on how to do this:

https://www.macobserver.com/tmo/answers/how-to-add-a-time-capsule-or-airport-extreme-to-an-existing-network-with-br

The interference would be in the form of decreased network upload/download speed or web browsing taking longer or even timing out. For example, you could do an Internet download speed test on your Macs comparing the speed with the Time Capsule wi-fi enabled along with your new router/modem, and then do a speed test with your Time Capsule unplugged/turned off.

Here are a couple internet download speed testing sites:

• http://www.dslreports.com
• https://www.speedtest.net

In an ideal situation your Wi-Fi network shouldn't be the bottleneck and it should strictly be going at or near the full speed of your Internet connection. It's just that having multiple wi-fi networks...especially if you aren't even using the wireless provided by the Time Capsule, would be pointless interference. Interference would really come into play though when you have more than one device doing something like streaming video.

But really this is all academic: if you turn off your Time Capsule that's going to be ideal assuming you have a very good wi-fi on the new router/modem.

Note: if it turns out your new router/modem actually has poor Wi-Fi compared to your Time Capsule you can also choose to turn off the Wi-Fi on the router/modem and just use the Time Capsule's wi-fi! That is Apple's preferred scenario. Some people might argue that's even better. It really depends on how old and worn-out the Wi-Fi is on your Time Capsule vs the quality of the Wi-Fi on your new router/modem. Certainly, managing your network with Airport Utility is very easy. And if your Time Capsule is several years old, and your new router/modem is very good then I expect it would be best to go with your new device for the Wi-Fi. But just keep in mind that doing it this reverse way is an option too (now or in the future should your wi-fi on your router/modem start to wear out).

It is a straightforward process to reset the network settings on the Time Capsule back to defaults. With the Time Capsule powered on, hold in the reset button for 7-8 seconds and release. Allow a full minute for the Time Capsule to restart to a slow, blinking amber light status.

Make sure that you have an Ethernet cable connection from one of the LAN <--> ports on your modem/router or gateway to the WAN "O" port on the Time Capsule.

Then, you can use Apple's AirPort Utility setup "wizard" to set up the Time Capsule again. The setup assumes that you will be using the WiFi on the Time Capsule, but you can turn that off after the device is set up if you don't need another wireless network for "guests" or other purposes.

When you click on Time Capsule, the setup "wizard" will take a few seconds to analyze the the current network, then display a screen that looks like the example that you see below, except that you will see a Time Capsule pictured instead of an AirPort Express.

You don't have to worry about stuff like "Bridge Mode". The setup wizard will automatically configure the Time Capsule in that manner.

Once you have the Time Capsule set up and running, try to run a backup over the new WiFi on one of the Macs. If the backup runs, you are all set. If you see a message that the backup disk cannot be found. then you will need to get into the Time Machine settings to "forget" the old Time Machine setup and then set up the Time Capsule again for backups.

Question 1: Is it workable to set up the old TC for backup(s) from both computers, wirelessly?

Probably. While it is possible to set up the TC in Bridge Mode to work with the WiFi of another modem/router or gateway device, the article that you linked below does not tell you that this type of setup is not officially supported by Apple.

So, while things should normally work in this type of setup, the word "should" is not the same as the work "will". So, the only way that you will really know if the Time Capsule will work on your network is to go ahead and try it out on your network.

While it is possible to change some of the settings on the Time Capsule, since you are trying to repurpose the device, your chances of success wil normallyl improve if you first reset the Time Capsule back to factory default settings and then set up up again. If you simply try to make changes without resetting the Time Capsule first, some of the old settings on the Time Capsule may interfere with the new settings and changes on the device.

Question 2: Should we expect to lose the backups previously on the TC or is there a way to have TC still retain the previous backups?

The TC will retain all of the old backups, but in order to get backups going again on the TC over the new WiFi network, you may need to reset the backup settings for Time Machine on your Macs.