Unfortunately, this is standard procedure.
Whenever you upgrade an OS, you have to check the apps that you using if they run under the new OS, if they need upgrades, or other conditions or inconveniences. Unfortunately, High Sierra has lots of those by introducing a new APFS file system (what could go possibly wrong there %$##@) and require apps to be 64bit.
Logic Pro 9 is one of those apps that won't run anymore in High Sierra macOS 10.13.
Please note, even if you pay the $199 for Logic Pro X, be warned that this requires a lot of learning and re-learning because a lot of things have changed.
Hope that helps
EdgarRothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
'I may receive some form of compensation, financial or otherwise, from my recommendation or lin