Although, if you have been using fpcupdeluxe, no idea if it has done some or all of this. Better check before overwriting.
Depends whether or not you already installed a (cross-)compiler on the first setup. If only used the install button on main window (for FPC or FPC&laz combo) then the answer would be no.
The good news is you can install a cross-compiler (tab cross) using the same install path in which case FPCUpdeluxe will update the existing FPC installation with the (selected) cross-compiler.
Basically all tha FPCup does is add the ppc cross compiler (when required/missing for the target) and the (cross-)compiled units to the units directory. Oh, and it probably updates the fpc.cfg so that it is able to locate the compiled units.
When done, selecting the target (cpu/platform) in Lazarus for a project should be enough though the used components will need to be (cross-)compiled for chosen target as well so that the first build for the project might be a bit slow.
Wen not using FPCUpDeluxe things are a bit more complicated depending on how FPC was installed and setup/configured. Without that knowledge all that can be said is to do the same as FPCUpdeluxe does but manually.
The wiki describes the manual steps that can be used but the order in which is done where and what to do with the result can be a bit cumbersome to understand for the reader (if you've done it once you'll most probably get/understand it). Always be prepared to ruin your existing setup (so make backups for the important bits in case you care).
