That depends on how fast you learn and also what you use for the Schematic end.
The PADS Layout and Router differences are minimal and you won't have a large learning curve. If you use the PADS schematic program, I assume the same is true although I haven't used it.
The main difference is DxDesigner. Here are the major changes.
Attributes are call Properties in DxDesigner and the keeper of all properties and what you can do with the is called a .prp file. If you wish to add a property to a symbol or schematic, it must be in the prp file. Luckily when you import old designs it gives you a list of properties that are not yet in the prp file.
The schematic editor no longer has a save button. Anything you do is automatically saved. This means you better make copies of a schematic before you go to play with it. Overall the schematic editor is nicer since it works more like a windows product. For instance, you can select multiple symbols and control property visibility or add properties to all selected items.
The way DxDataBook works is different. It actually now controls property visibility. However, there are new bugs like the "Remove Stale Attribute" control does not function in Hierarchical verification. This means all properties in your data base are added to a symbol during a hierarchical verification. Luckily, the page by page verification works fine.
Project configuration is also different as more control was added to what is called the .prj file. This file combines what used to be in the viewdraw.ini file with the .dproj file. It's sort of nice since you can edit the .prj file for items like you DxDataBook location, .cns, and .cfg files.
If you work in METRIC units, the same old bugs that have been around for 5+ years still exist.
The hardest thing is understanding what files do what. The conversion process of old designs is fairly easy as long as your hierarchy is clean and you top level schematic properly identified in you PADS2007.1 design. It works best if the PADS flow is selected in you original DxDesgner schematic before converting to PADS9x.
As CAD manager for our company I spend a good month understanding and checking features as well as updating coprorate configurations and procedures. This paid off as the EE's and PCB group were able to start using the tool with few major issues.
In my opinion, the upgrade was worth it. I suggest you go directly to the latest 9.1 release and skip the 9.0.x releases. 9.1 isn't that much better but if you are going to make the change, I would start with the latest version.