I'm not sure which version of espionage you are playing with. Sounds like its where you simply place agents on other worlds as opposed to having the permanent agents that only gather infoormation; since i think you can not take those off once you place them. Either way, Kryo is correct that once you achieve a level of intelligence you can't lose it. Hypothetically, if you had enough agents you could place them all on a civ at once and get high level, then move them all to another race the next turn. That would require quite a few agents though. See Loupidour for more info, he plays with armies of spies.

WHY this works this way may require some imagination as much of the game does. Think of it that the agent has made contacts on that race and can then leave and still get info. Or that he bugged the whole race. Etc.

One question, can I reach Advanced level (tech stealing) by placing agents on the espionage screen, or do I have to put them in the planets???

There are Four levels of spying for each race (as shown by the tiny row of 4 boxes) which can only be reached IF you have the entire area filled up with a complete spy group.

Say, if you were to remove the last spy (4th) for a race and move it to another, you wouldn't be able to *eventually* obtain the Advanced status unless dropped on that spot again for a good while **OR** by keeping an active 'regular' spy on a planet building for MUCH more time.

In previous versions we weren't allowed to remove a Spy (and to me, it made sense that such an activity should cost permanent espionage points rather than switching these around as needed!) from its static pool and it kept gathering info until a level was reached - but, now we can transfer this effect between races for a period of time spent there cost only. Weird.