You can subscribe to bubble FONT on the basis of an individual ARTIST and cannot change the ARTIST of the service you are subscribing to.
Also, even if you change ARTIST on bubble, your ARTIST on bubble FONT remains the same.
If you wish to use bubble FONT service of other ARTIST, you can cancel your current subscription and purchase a new subscription to the desired ARTIST's bubble FONT.
*FAQ > Questions on bubble FONT > Cancellation/Refund Inquiry > I want to stop paying for the subscription ticket. (How to cancel a regular subscription)