you should be able to plait it still. If you want to lengthen the neck, use more, small plaits. If shorten, try and make them bigger.
Also you might want to try plaiting from above the neck, and plait upwards, so the plaits sit up on the neck to make it look thicker.
(if you had a horse with a stocky neck, you would plait down more).
And don't forget to have an odd number of plaits along the neck (forelock makes it an even number).