In article , Sid
writes

How drastic can you be when pruning/sawing lilacs? I would like to take
away some old branches to let new ones take over.

The12'(ish) high lilac tree's very old but blooms well most years and gets
pruned a little annually after flowering (I keep it roughly the same
height). Main branches are about 4" dia, but there are new 1" dia ones
shooting up fast from lower down on these main branches. If i don't remove
one or two old branches (where the new ones start from), then i'll need to
remove the new ones as they're almost touching the old, and going in the
same direction. whats the best thing to do, and how old could a lilac tree
get before considering replacement?

I believe, but hope someone will confirm, that you should prune them in
spring if necessary.
--
June Hughes