Time machine copy of Mavericks for Yosemite

Each Mac can normally only "see" data in Time Machine for backups that have been made from that Mac......not another Mac. Your current Mac did not make the older backups that are still on the hard drive that contains the old backups.......so it cannot "see" them in a normal way.

This would be true even if the old Mac was using Mavericks and new Mac was using Mavericks as well.

There is a (long and complicated) way to see old backup files using the Finder on your Mac. "How" you do this depends on the method that was used to create the old backups.....whether the data is on a network drive, like a Time Capsule.....or on a normal external drive, like a USB or FireWire drive that is connected directly to a Mac.

If you want to try this, we can provide the steps......with no guarantee of success.....since Yosemite does not make this easy due to its own bugs.

We would need to know if the old backups are on a Time Capsule.....or.....on a drive that is connected directly to your Mac.

I would now like to start using Time machine on my Time Capsule for my MBA

That is fine.

What would happen to the Time machine backup of my MBP?

Nothing at all. The MBP will continue to back up just as it always has. Or, if you are saying that you have stopped backing up with the MBP, then the backups will remain on the Time Capsule.

Does it automatically get deleted?

No, you will have two Macs both backing up to the Time Capsule if you continue backing up the MBP. Time Machine will take care of everything for you, since it can easily handle 4-5 Macs all backing up to the same Time Capsule.

You can attach a USB hard drive to the Time Capsule and use AirPort Utility's Archive Function. This will make a complete copy of the data on the Time Capsule on the USB drive.

Once that copy has completed, and you have verified that the old files are on the USB drive.......then......you can Erase the Time Capsule drive and start over