Adjusting Ability Scores

I decided to make some characters to test out the character creation rules.
I see that prime requisite ability scores can be increased by lowering non-prime requisite scores. I know that this can be done multiple times, but can it be done even after the score has been raised to the point where the character would receive an XP bonus? Also, can it be done if the character’s ability score is already high enough to get the XP bonus?
In other words, let’s say I roll up a character with an Intelligence score of 12. I want to make her a mage, so I decrease her Strength score of 13 to 11 and raise her Int to 13, getting a +5% XP bonus. Would I then be required to stop, or could I further lower her Strength to 9 and boost her Intelligence again, to 14? How about if I had enough points in my other scores to raise my Intelligence to 18 - though it probably wouldn’t make much sense, maybe I really had my heart set on a mage. Would doing that be legal?
What if I rolled an Intelligence score of 16 to begin with? Would I be allowed to raise it to 18 by dropping points from my non-prime requisites?

"Once abilities have been determined, each player must choose a class. Each class has a prime requisite, and some classes have a requirement of a minimum ability score. If the prime requisite ability is high enough, the character will receive a bonus to experience points earned during play.
Sometimes, a player will choose a certain class and the character does not have a prime requisite high enough to receive the experience bonus. In these cases, 2 ability points may be sacrificed from an ability to raise a prime requisite ability 1 point. This may be done more than once, but no ability can be lowered below 9, and no ability may be lowered if it is also a prime requisite for the class, even if there are a few points to spare above the minimum required score. "
I read this to mean you could can trade any other stat at 2:1 ratio into your prime requisite as much as you want, even raising to 18. The 2:1 makes it inherently not a great idea to sack all your other stats for one though, but if you’re close to a bonus then it might not be bad idea to do it once or twice. The only restrictions are you can’t lower a non prime requisite stat below 9 and you can’t trade away from your prime requisite. So a magic-user could never make themselves dumber in order to get a bonus elsewhere.

I encountered this when making a character too. I thought you could only do it if you needed to meet the pre-reqs for a class (either I misread it or had an old version). Anyway I would still interpret it as “only if you don’t already get the XP bonus” so it sounds like some clarification is needed.

The intent is that you can raise your prime requisite(s). You don’t have to stop at 13. I’ll adjust the language of the rules accordingly.