what cny bikeman said is true
lbs techs seldom have time for experiments of idle curiosity
there is more money to be made by selling a whole derailleur
than by taking two hours to rebuild an old one

anyways
i seriously doubt this is possible
the main problem is that shimano derailleurs
are not designed to be servicable
beyond replacing jockey wheels and maybe bolts

so even if the derailleur parts mate together properly
which is a big if
it will be tough to get the thing reassembled and functioning

however
if you have the two derailleurs
you can go ahead and try
and report back here

also
what i want to know is
the reason you cant just use the ultegra derailleur

edit
also like cny bikeman said
a long cage will not allow larger cogs
for that you need a long body
like a mtb deralleur