Hi there!
No, there is no way to fix this. This is a behavior of the RPG Maker code environment. Every time you install a new plugin on your project, mine or not, you have a higher chance that you need to start a new game. This mostly happens when the plugin needs to store some information on the saved file. There are some exceptions, though. But it's hard to say.
So when you install a new plugin and load a previous save file that did not have that plugin at the time, you will need to start a new game. Keep that in mind, not only with my plugins but with others too. When you release a game, make sure that you already are using all the plugins you want.
When you mean healing in battle, do you mean at the moment a character is being healed? Or when it is regenerating health? By default no. You can try changing the font with a plugin command, but you will need to know the exact moment to do that.