Select all the library parts you want to change to another one. You can use Find & Select in the Edit menu to filter your selection. With Find & Select open, alt/opt+click on one of the library parts (GDL Objects) and then click the plus button in Find & Select and all like objects will be selected.
Open the object's settings box ctrl/cmd+T. Navigate to the object you want to replace them with.
Re your second question there is a translator setting to change all blocks to archicad library parts. They will not be parametric. You can, with some learning about GDL learn how to make them parametric.
Another method would be to export them from your Autodesk program to IFC and then into archicad. This should preserve the parameters. If you have textures attached in Autodesk then you will need to load them into your Archicad library for the object to find them.
HTH
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"