(I assume this is related to Ferrite.jl release announcements - #15 by kings94 and that you thus are using Ferrite.jl for this).
It is a bit difficult to understand what you are trying to do exactly, but my understanding is that you want to switch the boundary conditions mid-simulation? Based on the limited information in this post I think the easiest way would be to create two different constraint handlers with different conditions inside and apply the one you want based on the step.