removed location assignments dialog

That's correct. But the timing analyzor would give the actual timing on the different speed grades, even if they're with the same synthesis, and place&route result. If the timing analysis tells you that the new speed grade already meet your timing requirement, you probably don't want to waste time to do a total compilation again. The programming file can be used across diferent speed grades. Of course, if the timings are not met, then you would want to do a complete timing-driven compilation again.