If you’ve properly completed sweet spot base 1 and 2 at high volume, and you can handle that amount of TSS, then the general idea would be that you stick with the high volume. The final week of the SSB2 plan is a recovery week, with much lower TSS, so you don’t need to scale things down to a lower volume plan to get the recovery.
If you’re struggling with the volume of the plan you’re on then I would definitely switch down to mid volume - the high volume plans are really too much TSS for the majority of people.
Also, if you are new to indoor training - especially the FTP testing aspect - then there is a risk that you might have been able to complete the high volume plan because you under assessed your FTP. It seems to take most people several FTP tests before they really learn how to get the best of themselves in such a test. If though you are happy that your FTP really was set correctly, and that you managed to cope with the high volume plan, then jump straight into the high volume base plan.