This is part of the U-vs-O debate.
Chamoru? Or Chamorro?
Guåhu? Or guåho?
Since Chamorros are (in the main) used to writing in Chamorro, no system is going to be easily accepted by the great majority of people. An official orthography exists, but the majority of people writing Chamorro do not access it. Many who do have disagreements with the official orthography.
Of the small number of people who write in Chamorro (compared to the vast number of people who hardly ever write in Chamorro), many will write it as they hear it.
And the fact is that some Chamorros favor the U sound when they speak. Listen to them.
But a good number of other people, especially the older ones, favor the O sound when they speak. Listen to them.
I'm in favor of allowing people to spell it as they speak it, for the time being, because I don't think we have arrived yet at a commonly accepted orthography. We have an official one. But not a commonly accepted one yet.
But there is a bigger difference between a U and an A, compared to a U and an O.
Malessu'/Malesso' has a minor difference between them, in my opinion, compared to kampanåyu and kampanåya.
Call a guy fulånu is the same as calling him fulåno.
But call him fulåna and there may be trouble.