Well, now that I have two bikes the question doesn't apply.......
Anyway, with one bike I would certainly not be swapping to summer tyres yet, and the mudguards would still be on for another 4 or 6 weeks. Spring can be a filthy wet time of year, and may years we'll have a good hard frost in April. That's down hear in sunny East Anglia, too. One year in Leicester we had snow in June.
I left my summer tyres on too long last autumn, and I still have a lump from my fall in November as a result of that misjudgement, so err on the side of caution and leave those winter tyres on for a while yet. As for pressures: I run 110-120 in the summer (110 if I am doing a really long ride where I'd like just a little more comfort), and in the winter I am quite happy to plod around on 85 lbs. Smearing yourself all over the tarmac is not so much fun anymore, and slower tyres just mean you don't have to be on the bike quite so long to achieve the same amount of fitness work.
Mike