The value of an option to lock in an interest rate

Felix Salmon is wondering if he should lock in a lower rate on his HELOC. Seems like a good deal, but as seasoned experts here will know, there is a trade-off.

So the way I see it, my main cost of locking in now is that I lose the ability to lock in at a lower rate in the future. Even if the Fed funds rate stays on hold, the curve could invert further between overnight and six years, and the lock-in rate could go lower than 7.25%. I'm no expert in options pricing, so how should I value the option to lock in – the thing I lose if I actually do lock in?

Sounds like something I posted a while back. Unfortunately, it is not possible to refinance a house one room at a time.