How easy is it to migrate
That's difficult to say. If you had asked me 20 years ago I'd said "everything around a migration is difficult, in fact, I don't know where to start". Today, my main concern is to know if all the new stuff can be installed on the same old cabinet.
Can I get the same style modules so less moving about of wires
There're similar styles modules, with the same number of channels, and even you can find that it's possible to substitute two SLC modules by one of CompactLogix. However, more important is to check if the electrical characteristics are the same or else you will have to make changes in the external devices. For example, 1746-IC16 module (16 Input 48 VDC) has no direct equivalent in CompactLogix, so if you want to keep the external sensors you have to look for an alternative solution (adding a Flex adapter + 1794-IC16 , going to ControlLogix + 1756-IC16).
Rockwell provides Integrated Architecture Builder (IAB), a software to help with the hardware decisions. You can read about IAB in the following article:
https://theautomationblog.com/what-is-integrated-architecture-builder/
I use the IAB version included in my Toolkit and the wizards are an incredible help when I have to evaluate a migration: just describe the old architecture and the software chooses the best configuration for the target platform.
I have heard about a migrating software has anyone used it ?
Yes, there's a migration tool. As Ron said before, you can test the tool and then decides what path to follow. In my experience, automatic conversion tools are not perfect and just give you a point where to start.
In any case, you have to design your own protocol to audit the migrated application, i.e. you have to demonstrate that the migrated application works in the same way as the original did.
Finally, I recommend you to read this document from Rockwell about SLC to CompactLogix migration. Every migration's step is well described there:
http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/migrat-pp004_-en-e.pdf