Answer: To a large extent the answer to your question depends on what kind of sedum is growing there. The larger types can be dug and removed easily, but some of the smaller types are very persistent and would possibly need to be treated with herbicide containing glyphosate in order to eradicate them. If you do this, read and carefully follow the label instructions because spray drift could harm the lilac.
Larger sedums and the less invasive smaller types can certainly be interplanted with annuals. Larger perennials such as daylilies can also compete against the smaller groundcovering sedums. These more aggressice sedums will quickly occupy any nearby prepared soil. You might experiment and see what does well for you there.
However, there is something pleasing about a shrub growing happily in the midst of a continuous swathe of groundcover, and sometimes it is charming to think that a planting has survived for decades, from generation to generation. In addition, lilacs are large and greedy shrubs and can be difficult to plant under because of their large root systems and the shade they cast. That said, it might be a lot easier to plant annuals or other flowers in a separate bed.
I realize I have not given you a straight answer, but I hope this helps you decide what to do.
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