Well this is quite straightforward...

my Dragon Clan skills require a staff to heal
my Denden healer skills require a scroll to heal

when I change to my masterclass and start using scrolls, all my Dragon clan heal skills will be useless?

You will gain secondary weapon - in this case scroll which You will be carrying on Your back like Cit, so You will never loose anything, just gain new skills which will need scroll to use. When You will heal with Dragon clan skills You will wave Your staff, but when using Denden Healer skills You will take Your scroll from Your back and waive it instead.

thats reconforting. thanks Morfi