Anyway, unless you have some non-standard term sheet, I do not think you have to worry so much as long as you are honest and up front. If you approach this properly it won't hurt you or your company, it is business not personal. Many, maybe most, investors would take a better deal in 3 seconds flat if they got one before the final docs were signed and money wired, founders have to remember that and look out for their company and their team. I have seen investors back out after docs are signed but before money is transferred. Could a startup sue them, sure, will they succeed? Probably not and it will waste a shit ton of resources and some investors have counted on that.
Just my 2 cents, but don't try to back out of a term sheet unless you already have a new one in place from someone else on more favorable terms. But also remember, the terms aren't the only factor, the quality and capabilities of the investor should also be considered closely. I'd take a lower valuation from a better investor who I know is more bought in on the success of the company than getting a slightly better valuation with an investor that doesn't have much to offer outside of cash and isn't as bought in.