Hello Paul and welcome to the ArtRage forums 
Yes, in general that will happen.
If you want to change or add to an existing line of text, make sure that you have the text layer you wish to edit selected and the Text tool selected.
Then click within the body of text where you wish to make your edit and/or add more text.
To help keep multiple text layers under control, if I am using the text tool in AR, I tend to keep those layers in a group all of their own in the Layers Panel.
To make a Group in the Layers Panel, click the ‘Add Layer Group’ button at the bottom of the Layers Panel (it has an image of a folder on it) and then drag each Text Layer into the newly made group. You can also rename Groups the same as you can Layers to help identify what’s in them. You can also add Groups within Groups too.
Maker Of Replica Macoys
Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.14.6 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M