If I were, for example, to be on 12.04, but there were PPA's that did not have Precise releases, could I change the PPA entry to use Oneiric rather than Precise? Would that cause problems?
If my question is unclear (It probably is), feel free to ask me for more information.
It may work in some cases ,but in most of the cases it will causes broken dependencies,If you installed synaptic you can see red colour on such(broken) packages