Firstly, are bottom bracket shells always sat central on the frame? Or are they ever offset? Does the drive side of the shell always sit in the same place? I’d say no, and that BB shells are always sat centrally, in which case the end on the BB shell for a 73mm BB will sit 2.5mm further out from the centre line than for a 68mm BB shell.
In that case, if you use a 68mm BB in a 73mm BB shall the chain line will be 2.5mm out, which you won’t be able to correct with spacers.
Doing it the other way round may be possible using spacers, ie, using a 73mm BB in a 68mm BB shell.
Obviously, if my assumption that BB shells are always mounted centrally is incorrect, then this is wrong!