If the party membership changes, it requires a switching the macro out on each character and changing the label around in the code, which is the least amount of work I've found when dealing with switching in a party.
Honestly what i do in this 'group-changing' moments, is using mouse-over-healing combined with a UI-Mod, which let you sort your team-members alphabetical (like Pitbull or Grid), and mouse broadcasting to the healer. This has the benefit, that it is independent of the group-setup (as long, as the UI-positions is identical on your main and healer), and its no problem to change around teammembers, or to simply play some instances with friends.

In HKN this can be realized by defining a 'broadcast.mouse-position-and-key'-hotkey, like:

Code:
<HotKey g; shift g; alt g; ctrl g>
   <SendLabel healer>
   <ClickMouse NoClick NoRestore>
   <Key %trigger%>
   <RestoreMousePos>
and a macro bound to 'g' on your healer like:
Code:
/cast [mod:shift,target=mouseover] Powerword: Shield;[mod:alt,target=mouseover]Binding Heal;[mod:ctrl,target=mouseover]Penance;[target=mouseover]Flash Heal