Once the grass is growing, and the weather is fairly dry and stable - only because my summer field doesn't hold up well in wet weather, so although tempting, I'm holding off probably until early April.
If your field holds up to rain, and you don't want lots of spring grass then maybe swap now.
In our last yard we used to hold out until april too or you just end up with two very wet and and muddy fields both winter and summer. Beccause its been so dry here for the last couple of weeks, we have had ours out all the time in the winter fields but we have set up hay feeders so although there is no grass they do have stuff to eat constantly. I think its suppose to come in to rain again weds so they will come in again over night as it gets so rough pretty quick.