Are these grafted?
If so sprouts growing from the rootstock will not be 'Ivory Silk'.
If not you are good.
All you can really do is get them into the ground and let them grow as much as they want. When you are absolutely certain that parts are dead (into June at least in your zone) then prune those parts out.
What you end up with depends on whether or not these are grafted plants. But I do think they deserve a chance to live.

Thanks for your reply! Upon further inspection, they do look like they have been grafted. From the very little I know of grafting does that now mean there is a possibility/probability that the 'standard' it was grafted to is the more common lilac bush? If so, this may be the first time one of my goof-ups end up being a good thing! I have been so desperate for lilacs for such a long time that when I saw these at a very good price I snatched them up. When I realized they were actual trees I was a little disappointed but quickly came to appreciate how beautiful they are in their own right. If they turn out to be the type I had actually wanted I will be thrilled! I will also be on the lookout for another 'Ivory Silk' since I fell in love with them despite my naïveté! Thank you!