When you migrate to HPOS you can keep your site in compatibility mode while testing.
This video takes you through the testing and compatibility mode settings of HPOS.
Video
Wrap Up
Leaving your site in compatibility mode defeats the point of HPOS, migrate over and get out of compatibility mode as soon as you can, and don’t forget to clean up the legacy data.
If you need help migrating your site to HPOS I have a fixed price done for you packaged service. Get in touch for details.