In CRM 2011 I was able to alter form tooltips using DOM manipulation. Now, I'm using CRM Online so I can't do this any more. Is there a way to change a field's tooltip dynamically using javascript?
As far as I know there is no supported method is set the tool tip with JavaScript. I don't believe this was supported in CRM 2011 either.
New capabilities to replace unsupported methods, makes this clear.
Show custom tooltips.
Edit the attribute description. The attribute description text will appear as a tooltip. No code required.
So the best you can do is either go unsupported (I wouldn't recommend that as Microsoft might change the DOM and break your scripts). Or use another approach, how about form notifications?