Been there - done that !!
I joined BA as a DEP on the - 400 a few years ago, having been up to then a Captain on A320. I guess I joined because I wanted to prove to myself that I could get into BA and because I felt like a change of scenery. I was fully CAA rated on the - 400 with about 4000 hrs in the right seat of the Classic and -400 combined. My rating was still valid, i.e it was less than 5years since last LPC had lapsed, but BA gave a complete type rating course to everyone, followed by about 16 long haul sectors to check out. SO WHY DO THEY WANT - 400 RATED GUYS, if they're going to do the full course all over again?
Anyway, I only stuck it for about a year, as under the bidline sysyem the junior guys get ALL the crap, i.e. endless standbys and no stability. Plus the money was crap compared to what I could make on contract as a A320 Captain.
So, my advice is: if you want to get a - 400 rating for free and try longhaul, definitely apply. It'll be a feather in your cap if you get in; you will have proved to yourself you're as good as 'the best'. But if you want to have some some chance of a command before you die of boredom on 14 hr sectors, don't.