I think it's going to depend on 2 things: how big were the planter boxes? and were you careful to have the components damp when they were put together? I've never tried anything smaller than a 24" pot, but I would expect smaller would dry out faster, meaning you would have to be careful to keep checking for moisture. I also incorporated a layer of garden soil that had a couple of earthworms - I know, that isn't a normal component - but I really think that the worms speed up the process :) but that pot did decompose over winter except for a thin layer right on top.

If everything was damp when you began, and stayed reasonably damp through the winter, you should have stuff that is almost compost by now -- you can plant into it directly with no difficulty. But do poke around to see if it had dried out and didn't decompose well -- if that is the case, dump out the partials mixture, then start again using a couple sheets of damp newspaper, an inch of partials, a 1/2" layer of brown if you have any (dried leaves, etc), a couple sheets of damp newspaper, a thin layer of green (grass clippings, etc) and/or a sprinkle of bloodmeal; repeat the layers up to the pot rim, and then mulch the surface with shredded leaves or cocoa shells or bark fines. Fresh grass clippings are considered a "green" and not usually used as the top mulch -- dried grass clippings are a "brown" and make a fairly good mulch if spread thick enough. Most annuals are quite happy to be planted directly into the raw lasagna -- just trowel the hole and then surround the rootball with an inch of compost or potting soil. Check daily [or set the water emitter] to be sure the soil is barely damp at 3" (which is probably your convenient fingertip). Do remember that even lasagna layers made with partials and/or earth will shrink a bit as they "cook" so over the summer so you will need to keep adding a thin layer of damp mulch on top.

By planter boxes, do you mean raised garden beds or little boxes?
As meldy said...success depends on the size of the boxes and the decomposition that has gone on during the winter.
If you kept it moist, you should have a decomposed mass of compost and can just dig a hole and plant. If you didn't keep it moist, you may have a layer of newspaper with some dried grass on top.
Dig a hole and see what's down there.
Linda C