For me, not sure .5% would be sufficient to offset the flexibility, and you are already at a rate that is very likely significantly lower than what the market will return. But your timing to RE could change that -- we refi'd to a 15-yr largely because the payoff date coincided with our planned RE date, which lowers the nut we need from that point on.
I'd also look closely at the "no cost," as usually that means they just roll the various costs into the new mortgage. Maybe there's no origination fee, but there are always filing costs, tax costs, title insurance, and other costs associated with a refi. Get a complete quote for all of those costs, and then compare the overall savings vs. the option of just prepaying your current mortgage on the same schedule.