Keep the old effect and turn the new effect into a new spell.
Do this and I'll keep my paladin around.
If your out of the loop, the old Beacon of Light says:
The target becomes a Beacon of Light, healing all party or raid members within 10 yards for 2000 over 15 sec.
The new Beacon of Light says:
The target becomes a Beacon of Light to all targets within a 40 yard radius. Any heals you cast on those targets will also heal the Beacon for 100% of the amount healed.
Is it really too much to ask for both?