Can the new ArcGIS Pro use existing ArcToolbox scripts?
Or to put it another way, will I have to rewrite my existing ArcMap toolbox & python scripts if I want them to run in ArcGIS Pro?
This from ESRI:
Your existing toolboxes, models, and scripts can be reused in ArcGIS Pro. However, if you use arcpy.mapping, you may need to make adjustments to work with project files (.aprx) and maps, instead of map documents (.mxd). In addition, ArcGIS Pro uses Python 3, so it is possible that you may need to make some changes to your scripts.
There are still functions not available in ArcGIS Pro so if your script uses them then I would guess you are out of luck until they are added. For instance, Data Driven pages is not available.
Possibly, as long as your toolbox is totally contained with ESRI tools. If you are using any external scripts or script-blocks with Python then you will have issues as the new platform uses a newer version of python which does have a changed syntax that will break.