If you are trying to port code that relies on 32 bit that is a big mistake and you just are caught out.....
Ideally, to port your 'Lazarus' code from 32bit to 64bit you need to change CPU Target, the widget set (Carbon->Cocoa) and the compiler. Three settings and build.
Its likely however, you will find some things that work a little differently but more importantly, you find some things that don't work as they should yet. Someone taking this approach is not dumb, they simply had no choice. Six months ago Cocoa was unusable, today, its amazingly close !
Right now MISV, I know of two things not working, Memory leaks in PageControl and Printing. Bet there is more of course but thats hardly surprising.
In the category of things that are a different, requiring some coding around, only one. In Carbon, I could hid my main window and users could still see its menus, sadly for me, in Cocoa the menus are disabled when their main window is hidden. I don't know which is correct behavior so cannot report it as a bug. My problem to fix my code.
So, start testing your app, make a mode thats for Cocoa and test away.
We have seen massive progress with Cocoa rcently, its very nearly usable and the folks responsible are doing a fantastic job, thanks !
Davo