Back in the 3rd edition or before, I would have said "No. Congratulations."
But if the system of guideline and modifiers used in the 5th (and 4th) edition(s) is to make any sense, I'd argue that the answer would have to be "Yes, the level changes as well."
Essentially, if it does not, we'd soon have a system with so many exceptions that the core system would be largely irrelevant.
That makes sense (though I could see a case for "it's lower, but you do t know why, and can't replicate it"). Still, how does that effect the time needed to invent, or whether you can even learn the spell at all ?
Agreed. But there was a thread about replicating the results of experiments recently, and I must admit it only cemented my opinion of this matter.
HOUSERULE: I'd probably let you get that "for free", meaning that you'd research the spell as designed, but when done, discovered that it didn't work exactly as planned.
But that's really up to your troupe
Suggest that the Magi's LT stays the same, but the actual level of the effect is +1 mag higher than expected. So the spell gets written up at level 20, even though it was designed as 15.
i.e. You get the extra +1 mag for "free" but another magus recreating it later will need to craft it at level 20. Likewise somebody learning it has to learn at level 20.
"The spell or effect’s range, duration, target, or potency is increased." >> given you'll need to decide what this boost means, thrash it out with the GM. Altering a R,D,T upward could actually be a hindrance anyway for some spells. But more powerful in terms of affecting a slightly wider array of targets is cool and seems good.
The magus invents the spell, this time, as a level 15 effect (or whatever), but what he has created is an effect one magnitude higher (for example, level 20). So, the lab text is for a level 20 effect, he casts it as a level 20 effect (which may be difficult for him), it is resisted as a level 20 effect, it is detected via InVi as a level 20 effect, etc.
However, if the result happened when investing an item with an effect it would only have the vis cost of the original intended effect. So a level 30 effect (which became level 35 via experimentation), would still only require 3 pawns of vis and would only use 3 capacity slots in the item. But, if the effect was subsequently reinvented from the lab text (to invest in another item, say) it would be as a level 35 effect with the full-cost.
It makes sense that the vis cost (and therefor 'space') of effects in a device is unchanged. Because you can also Experiment when Investigating a Device, and if you suddenly change one of the effects it would make no sense that the effect now cost more vis - which nobody can 'pay' - and the device may even be unable to hold this many levels of effects.
As for changing the level of a spell being invented, what if the new level suddenly soars above level 50? Does it become a ritual? That would often be somewhere between annoying or devastating for the usability of the spell.
If no spell parameters reach Year or Boundary but merely the level takes it into Ritual country, I may be tempted to waive this clause. But then again, no. Because the consequences would - as earlier posters mentioned - mean exceptions to the rules. And once the lab text is written this means non-standard spells deviating a lot from the way magic works come into circulation. Better not!
So if your experimentally invented 10th magnitude spells gets this kind of boost it's back to the ol' drawing board.
..that's when the Bonisagus clean-up crew appear, gag and stuff your magus into a sack, wreck the lab, and blame twilight (ooooh spooky). nothing to see here covenant members, look into the neuralizer, move along, move along..
It could also be the justification for the start of a major original research for hermetic theory too if the character and game suited that style of play.