Probably covering it with the plastic delayed the decomposition, though unless leaves are ground up they can take some time to break down. That said, moving them aside and planting will probably work without a problem. It might take the soil a bit more time to warm up - though the black plastic should have taken care of that issue. Ruth Stout used to cover her entire veg garden with any organic matter she could get her hands on, and never tilled it - just added more mulch each year, pushed it aside to plant seed or seedlings, and pushed it back around the plants once they were up. (The Ruth Stout No-Work Garden Book)

-Rosalinda

The plastic probably kept the leaves drier and as Rosalinda said slowed down the decomposition. We use that method to overwinter semi hardy plants and keep them drier over the winter. The black plastic will make the soil warm sooner. You can leave the leaf mulch in place and just move it around to plant. It will eventually decompose. Hasten it by putting some top soil or compost over the leaves.

Penny