Not sure if this is even why you were asking, but you can't overwrite a later raid CP with an earlier one. For example, if you have Gorgons CP but join a Templar game and wipe, you won't keep Templar and will still have Gorgons (unless you've restarted the raid progress from the mission selection). So doing abyss-bridge won't erase Crota.
fyi- it's impossible to join any activity if you are more than 3 levels below it's listed level (i.e. Lv30 is minimum to even join a HM CE game), since 4 levels down means all enemies are immune to your damage. A 27 can only do NM CE since it's listed at 30 even though enemies increase to 32 by the end.