just to confirm (with links to relevant confirmations)
as you have discovered, you cannot change the reservation once you have checked in with ryanair
Can I change my booking after I have checked-in online?
however, with easyjet you can
Can I change my flight details after I
as the previous poster says it might be cheaper to book a new ticket rather than amend the existing.
from LPL you only have FR as your choice of operator to DUB.
from MAN you have a choice of FR or EI
With EZY you don't seem to actually check in. I booked a flight yesterday BFS-LPL and when I had completed the booking it then gave me the option to "Print Boarding Card". Simply making the booking appeared to have checked me in!