Eldritch Master prestige class

[Yogibear41]

The Eldritch master is a prestige class for spellcasters from dragon magazine #280 that does not advance spellcasting in the traditional sense, it gives bonus spells known, and every couple of levels lets you cast a spell of 1 level higher than the max level you can cast.

My question is say you return to your original class after picking up one of these "may cast a spell 1 level higher than he can cast abilities" does this spell slot get moved up to the next slot if you advance your casting the normal way after gaining the ability?

For example a 5th level sorcerer/3 level eldritch master gains the ability to cast a single 3rd level spell, if said character then went back and gained a 6th level of sorcerer, would he then be able to cast a single 4th level spell once per day?

[Fizban]

Depends on how its phrased: a slot is a slot and doesn't change unless the ability says is does. You can quote the relevant paragraph for rules-lawyering, I'm sure people will have different interpretations.

[Yogibear41]

"At 3rd, 6th, and 9th level, the eldritch master gains the ability to cast a spell one level higher than his current highest level spell once per day."

[Fizban]

The combination of "at X level" and "current" in the same sentence means I would rule it as a poorly worded single spell slot, fixed when you get it. Since you are quite explicitly casting the spell from that ability, each new iteration does go a level higher weather it works as a spell slot or only as a castable spell. So assuming a 10 level class, it effectively grants 6/10 casting progression, if you consider a single spell as progression. I assume it grants a bunch of lower level slots or other abilities as well.