Re: In the moment....transitioning into another technique
Yeah - what ChrisHein said. There are many bad ukes, from mukyu to the dan levels; it is easy as uke to let your preconceived notion of a technique stop a working technique in its tracks. As nage, part of what you're learning (and what's the most difficult to learn) is leading uke. Techniques are not just reactive, i.e. you are not only going with the flow, changing when uke changes - rather, part of trying to master a technique is creating a pattern that uke falls into, so that uke is reacting and you are leading. You may feel like uke is fighting you (sometimes they do, sometimes they don't), but if uke doesn't feel led, that means there's something you need to work on. I know, the first years are frustrating! (So are the ones that come afterwards)
That said, every once in a while it's fun to switch up on a particularly recalcitrant uke, just to keep them awake. Unfortunately, that's always when sensei will look over at you and see that you're not doing what s/he taught, and then it's "yes, sensei... sorry, sensei... yes, sensei.... thank you, sensei"... <sigh>.
Better to push through, and think about where the flow of leading uke is getting clogged up so that they feel like they should fight you...