There is not an upgrade path from pro to scratch pro anymore. You have to buy scratch pro and sell pro to recoup the cost. This has been covered on the forum before as well.
If you want to use an audio4/6, you could get a copy of duo/duo2 and then do the upgrade to scratch which is a little bit less expensive than going with the audio 10. You can get TSP2 with the audio 10 for 600 bucks on amazon though, and it’s hard to get it much cheaper unless you get TSP1 or TSD and then upgrade to TSP2.