Answer: If you do this, the best time to cut them back is in the spring. It would also be better to cut them back to the ground rather than to two feet. This will encourage new shoots from the roots which you will need to selectively thin. Lilacs by nature tend to look a bit wild, so don't expect a perfectly uniform result.
Another possibility is to renewal prune over the course of three years. To do this, remove one third of the oldest stems in year one, half the remaining old stems in year two and the rest in year three. Also remove any shoots smaller than a pencil each spring. Do all pruning at the base of the plant. This way you will have all healthy new wood within three years. This is best done in spring immediately after bloom, or even earlier if you don't mind sacrificing the blooms.
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