Very easy to switch, in my experience. It is like muscle memory. It might take a few weeks to get comfortable with manual, but once you do, it doesn't require much thought to switch gears.
It's a sequence that just happens, similar to typing. You still need to think about what you're saying, etc., but you don't have to consciously think, "now I'm typing an S."
I ride a motorcycle. In the motorcycle version of Driver's Ed, they taught us to simply think of changing gears as the same motions you'd make to wring out a wet cloth. Thinking that way automatically improves the timing and flow.
Once you've practiced enough, you simply stop thinking about any part of the gear changing steps. You don't even need the speedometer or tachometer when gearing up or down anymore because you already know how it sounds and feels. It becomes a subconscious process.
But that initial visualization of wringing out a washcloth makes it really easy to figure out. Soon enough, you have muscle memory and don't have to think about it - you just do it.
It's a sequence that just happens, similar to typing. You still need to think about what you're saying, etc., but you don't have to consciously think, "now I'm typing an S."