romello
Hi
If a template has its own module position you can not modify them via page builder, you can add own style over it, like adding a block and call the module in that block. This way you can load the module again in builder.
T3 does not have any builder yet, the older builder is stand alone builder, it would not be auto upgrade as both are different, It can be export and import pages in new builder.
Our team is working on it to make it ready, its very early at this stage to say a final release date as its in preview.
Let us know if there is any doubt or questions.
Regards