depends on the size of the system. To generic of a question.
New for 7.5
1. You will need signaling servers
2. Your numbering scheme will change if you are an Option 11C
3. If you are an Option 11C you will change from SSC card to MGC cards and seperate processor card
4. I don't even want to mention the introduction of System Manager/Session Manager and UCM.
5. Then there's the whole "We want you to go VoIP so a TDM solution will be much more expensive than a VoIP solution.
But you have to ask yourself, "What will I get with the new system?" Is there some application you are needing that you will get with the upgrade?
If I had to start with 50 phones, you are better served replacing with an IP Office and getting the trade-in on your system, but you move from a PBX to an advanced KEY system. Although the IP Office is not a bad replacment and it has some great features.
If I had to guess on an upgrade based on a 4.5 to 7.5, I would guess in the neighborhood of $100K for the software and hardware, then another $10K for the installation, and that doesn't include your other equpment like voicemail and any call cetner applications.
Hope this helps.