change from 32bit to 64bit

There isn't a change to make.

What you need to do is figure out if the developer of those apps have an update, or if you can replace those apps with better ones.

If you can't, then don't upgrade to Catalina in the Fall.

Most apps were updated to 64 bit long ago but you may have some that have not. You can check by going to Apple menu > About this Mac > System Report > Software - Applications and look in the 64-bit column there.

If you have some where it says "No" in the 64-bit column, it is likely that the developer has an updated version available, although there may be a fee.

This is what Apple says about this: 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Fairly complete FAQ

https://support.apple.com/HT208436

Christi2844 Said:

“...will my current apps work if I make the change?”

Request a New Installation File:

FWIW, just contact the software developer, and ask if you can do a swap out for the software type (32- or 64-Bit). It would be the call of the developer as to whether or not you can swap out the drivers.

License Keys and Devices:

Most software nowadays that is not freeware would provide you with a license key card, and the license key would be valid for only a certain number of devices (Typically 1, 3, or 5 devices).