OK, here's an answer from a UK approach controller - i.e. TA/TL is day-to-day business. It's a personal view so you may get other answers from other posters!
I know what the UK rules are. I know that other States have different rules. I know that some SOPs do not adhere to UK rules whilst within UK airspace. I am aware that some pilots do their own thing regardless of the rules (and, sometimes, SOPs). In short, I am very aware of the issues involved - both for controllers and pilots.
Consequently, I will never clear an aircraft to climb to a FL knowing that I am going to want to stop the climb at an altitude. Frankly, if there's the possibility that I'll need to stop the climb because of other traffic then I'll only clear the aircraft to the safe level and then, as it approaches the level change, I decide whether I can clear it further. Same principles apply for descending traffic.
Some may say this is inefficient but it's fail-safe - and I've been in this game long enough to know how easy it is to get distracted.
But stuff happens and plans sometimes have to change. Working around the TA I'm always aware of the relationship between the TA and the next safe level. Therefore it is fairly easy to work out what level (be it a FL or altitude) will be safe relative to whatever the problem is if it really is necessary to change the plan. I'll use whatever level datum the pilot should be using according to UK rules first but if there's any hesitation or question I'll have the level referenced to the other datum ready. If it's all turning to a heap of manure there's always the 'stop climb now and advise the level' and I've got the mode C to help also.
In truth, particularly where the TA is 3000ft or 4000ft, it's not a common problem because it so rarely happens that a clearance needs to be changed whilst an aircraft is climbing through altitudes because it happens so quickly for most commercial ops type aircraft. Perhaps it's more frequent when descending but I haven't seen it done often. Of course, if TA is 10,000ft or 18,000ft it's going to be a different story.