Hello Colin Malcolm

you cannot switch only the SQL Server Version.

If you need WinCC flexible 2008 SP3 you have to get sure first if you Operating System is compatible with WinCC Flexible 2008 SP3 -> because Win10 or newer is NOT released and compatible with SP3! (only with SP5)  LINK

So if this fits, you would have to uninstall WinCC Flexible complete and als the SQL Server of this System and then you can install 2008 SP3 (its because this version is only released with specific SQL Version -> check in Compatool)

Bye Murof