What's the real reason you want to replace them anyway? Sometimes the best answer is the easiest one.
Good question, and certainly a valid one. Hence why we're justifying replacement.
Our current performance is fine. We are an all gigabit network with 10gig connections between the switch closets. We've never *really* stressed our switches that I know of, and I'll frequently get full wire-speed between systems.
1. We're looking to replace our aging phone system with a VoIP system (internal or hosted), and we want to use PoE, which our current switches do not support (and it's not an option).
2. We're looking into virtual desktops (moving to 100% virtualized), so we expect our bandwidth needs to increase.
3. The stacking model of these mid-tier switches leave a lot to be desired. If the "primary" switch fails in a stack, you have lots of problems. Or, if the switch with the connection to the other closets fails, you have more problems.
4. Have you ever tried to replace a fully populated 48 port switch in a fairly dense rack? Yeah, it's doable, but it's a pain and take a lot of time. You sometimes even have to take the rack mount brackets off just to slide the switch out of the rack. Yes, we could possibly optimize our rack layouts, but it's hard to find a home for full-height PDUs.
5. There's no redundancy. Dual power supplies was an option (which we did not purchase at the time), but otherwise nothing else is fault tolerant. The HP switches we're looking at are fully redundant (management modules, switching fabric, power, etc).
6. We already have a few failed ports, and we expect more in the future. (Don't get me wrong, the 3Coms have been great switches! Our needs are changing and I'd like to build it out for 10+ years of use).
7. Unlike everything else around here, we actually don't have support on the switches anymore. Not sure why. But, it's nice to have the option of support if we need it.
8. We want to implement NAC (on the switches, not in 3rd party software), and these seem to have a nice implementation.
9. The 3Coms are not secure. Our security auditor can take them down/DoS them in a few minutes. The HP's have protection against that, and also have basic virus protection for the endpoints (monitoring activity on all ports).
Like I said, the documentation says MTBF is 21 years or so, but who are they kidding? I'd like to know real-world lifetime expectation of this equipment before you start to have more and more problems that start causing downtime. We have about 5+ years on our switches (maybe more for a few of them), and they've been solid, but I don't want to have to start to worry about them and spend time fixing.