"fcl-db" is part of fpc.
The lpk package file is not used. The units are compiled once when fpc is build via its Makefile.
You have to rebuild fpc (using the Makefile / you may have to download fpc sources from git / not sure if the Lazarus install contains all the files needed for a rebuild).
Even if you force-fully rebuild the package, it won't work. Because the resulting ppu will be in the wrong location. And also because any other depending fpc package will not be recompiled.
In some cases, there is a workaround....
---- EDIT: this also suffers from the non debug ppu still being in path - so likely not working...
If you use no other fpc package that includes fcl-db (including not using any lazarus or 3rd party package, that in any way uses any fpc package that uses fcl-db)....
You maybe able to make a copy of fcl-db (and rename it, or you will likely have a whole new lot of troubles), then open that package and recompile it.
Then find any package that uses fcl-db, remove fcl-db from its dependencies, and add your copy instead.
It's still a lot of work.
It only works if all pre-conditions are met.
And you must change some dependencies, which you later have to restore.
iret's idea may also work. But you need to add the path to each package that needs it. And playing with paths is also of the category "you better know what you do".
Mind that you can't easily remove the fcl-db ppu from the include paths. So the may creep in, and cause trouble.
You can easily get a debug build of fpc, if you use FpcUpDeluxe.
At least on Windows it is easy enough to switch between different fpc builds.
On Linux and MacOS that requires some extra steps. Not sure....