You really don’t even need to respec man. Just make sure you get the best healing gear possible.
What do you gain if you spec Holy?
12% Holy Light and Flash of Light healing bonus.
5% more intellect.
Divine Favor (free crit heal because it requires you to spec 5/5 Illumination; which gives you mana back following a critical heal).
And 5% Holy Crit.
These definitely pay their dividends on the parses and when gauging your ability against other Paladins of equivalent gear, but by no means will it be far worse healing.
I solo healed UBRS with just prebis back in September with my full Ret spec (32 Ret - 8 Prot - 11 Holy).
MC and BWL are very straightforward and require minimal movement from healers.
Your main job is keeping tanks alive. To do that you spam Flash of Light. Maybe Holy Light once in awhile. And Lay on Hands if you’re panicked at the end of a progression fight.
Depending on the ability of your guild, you could even do the Ret thing for the entire duration of Classic.
Here is the caveat;
You are a bastardized and usually judged spec. Due to the fact Ret does not compete on the meters in any capacity, your talents will constantly be called into question. If you can deal with the backlash and are a team player (letting Warriors and Rogues and even Hunters or Druids take pieces you want first) you might find yourself in a position to stay Ret.
Be the guy that everyone wants to play with. And if they ask you to heal, don’t grumble.
I wrote a response before Classic started to a question about this. Take a minute to read it if you don’t mind. It will help you know what you’re getting into;
As for myself, I went Holy the moment the honor system came out so I could be in the position I am now at Rank 12. Ret is fun, especially in PvP or when you get to have a fun raid and get power infusion (looking at Esfand here). But remember your primary role will always be a healer to the vast majority of players.
Good luck brother of the light.