I agree with Hesper. It took me a while to get comfortable with this concept, but it's really no big deal if done correctly.
My family had a 1980 O'Day 28 when I was a kid. Based on your description and my somewhat blurry memory, I recall the upper and the aft lower went to the same chainplate, then the forward lowers were on their own chainplates--is that right?
If it were me, I'd do the following:
1) Run the main halyard to the starboard stanchion base or something else solid nearby the chainplate and tighten (enough to support the mast, but don't put too much load on a stanchion base).
2) Loosen the port upper just a bit to be roughly even with the starboard halyard tension, and loosen the port lowers a lot since you're taking the starboard lowers off (without anything to take their place).
3) Then I'd can remove the shrouds on the starboard side and get to work.
Does that make sense? I've done this several times on our current boat with no ill effects. If the mast was deck stepped, this might be different. Hope this helps!
1984 Sabre 34 Mk I