Okay basically the title says it all but i will elaborate.
Unohana we know was not to long ago the captain of the healing division in the gotei 13, but we have recently found out she was the 1st kenpachi. The manga never mentioned if she had the ability to heal 1000 years ago, all that was stated by was that she adapted her abilities to heal her during battles so she never lost a fight.
so my question is did she change her zan's ability?
If owner could change zanpakuto I guess it'd have to require second Asauchi. Her Shikai represented the shell, that she created, but her Bankai was reflecting her real, murderous side.
I believe she could simply develop new techniques that are focused on healing, and not use the killer ones, instead of replacing the latter with the former.
Unohana said she learned how to heal in order to be able to keep fighting... Considering her techniques were taught to her by a royal guard I would think it is well possible she learned them well over 1000 years ago. Or are you saying her zampakuto has the ability to heal? Well, her shikai has some measure of such abilities but we don't actually know what the ultimate purpose of her zampakuto is supposed to be. She did use her bankai to actually fight zaraki at full power so I would think its purpose is away from healing...
Anyways, as far as a shinigami changing abilities, I don't think it is possible. Abilities are supposed to be innate at large. Some details regarding how zampakuto are made within a shinigami have yet to be elaborated upon however I even get the impression that it is possible for abilities to be ingrained within a shinigami's soul even before a proper zampakuto is formed within its soul. Developing new techniques should be well possible IMO, via making different applications of the already existing techniques, however changing the abilities of a zampakuto altogether should be impossible.
Or it is simplier. She didn't use Asauchi until she fought with Kenpachi. Then she changed, wanted to learn healing, was given an Asauchi, thus the healing Shikai and killing Bankai.
Lets take Aizen, for example. His personality is basically deceptive; he manipulates people and sneaks around, as seen during the TBTP arc when he eavesdropped on Shinji and Urahara's conversation. As a result, his core abilities are all related to deception; without a Zanpakutou he can alter an individual's perception slightly, whereas with a Zanpakutou his powers are manifested on a greater level and he can actually hypnotise his enemies).
Now, if Aizen took a change of heart and his personality changed, his core powers would also be altered. If this happened, I'd imagine his Zanpakutou's abilities would first become less potent as his Asauchi is 'desychronized' with his powers. However, over time the Asauchi would catch up, and would begin to manifest the new powers.
Well doesn't her Bankai more or less melt everything around it? I assume she had to learn to heal herself otherwise her Bankai would kill her, would it not?
I actually don't think that there abilities change. I think that the more accurate way to discribe it, would be it evolves... If i remeber correctly, Yamamoto indicated in his fight againts the sternritters leader (who ended up being afake anyways) that he couldn't take his bankai because they didn't know how it worked anymore. But it didn't necessarely change completely, it just improved and evolved on what was already there...
As for Unahana's, she was ruthless and the master of countless sword style. A killer like no other, but it seems that she began to change little by little before she got her asauchi and that shows in her abilities. She has a healing shikai, and it seems like she has a murderous bankai. Which from what we have hard about her, is pretty accurate... The healing aspect comes from her want to fight longer, but that still only supports her main focus to want to kill longer. Hence her banka supports her main focus of fighting style.
And in saying all this, i can see ichigo having a new and slightly different ability. But still relating too throwing energy around with his blade. (blades now)
Well there's an instance where the handsome subordinate of Zaraki always call his Zan's other name and thus it will change into multiple blade type zan since he is embarrased to have a magic type zanpaktuo being from the fighting division of Zaraki.... So i think that's the only way... Calling it on another name is practially changing its abilities...