It will sell out, but it will be closer to the race date. This year, I planned my training schedule around it, but delayed in signing up. It sold out the morning I was ready to fork over my money (early February...if I remember correctly). This was about a month before normal. I switched things up and did NOLA 70.3 the following week and actually and glad that I did as I think that turned out to be a better race (for me).
Galveston has typically been a tune up race for many doing IMTX. Because of the date change next year for Galveston I don't think it will sell out near as quickly, if at all. Of course, I wouldn't take the chance and you should register sooner rather than later if it is a race you really want to do.
I love this race but won't be doing it next year with it being just 3 weeks before IMTX.