PSR
Your understanding is pretty good and the earlier posts are either showing remarkable lack of basic knowledge, or trying (as your case illustrated) "to cheat the system".
"Switchover" altitude partly depends on atmospheric conditions, but more on what the mach / IAS specified are. In an A320 series, at FL350, if you ask me to slow to min/holding speed, I am effectively flying IAS at FL350.
My (and I am unusual) main concern with ATC speeds are unrealistically high speeds required at lower FLs, particularly "increase speed" instructions. These cost a lot of fuel, and given we still fly in random directions / hold, achieve nothing!