I have an L72S that I have tried to convert to an L82S. It's been an adventure and I'm still not there.
I was thinking that I was going from a CLX processor to another CLX processor - no problem because I've gone from L6x to L7x processors several times without any issues. Our local distributor helped to restructure my mindset. He pointed out that an L6x and L7X are just different hardwares(processors). The L8X is an all new beast. It is so much faster because it has mutiple processors AND because it handles instructions differently. The software is different too. So you need to think of this conversion as something between a L6x to L7X conversion and a PLC5 to CLX conversion. Not quite as easy as the first and not quite as hard as the second since the IO hardware is the same.
There is a 158 page document for replacing an L7x with an L8X processor. I scanned thru it and thought I picked up the few details that mattered to me. I was wrong - I needed to read in much greater detail. Here's the link.
http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1756-rm100_-en-p.pdf
I got my program to convert from an L72S to an L82S (ver 31) without too much problem. When I loaded it in to the processor, it seemed to work - at least it didn't puke. However, it was difficult to get online sometimes. We ultimately found that our Wonderware was using the primary IOServer and Backup IOServer at the same time. The two IOservers were barraging the processor with too many messages. Apparently the L7x could handle the excessive class 3 messages by taking the extra in as class 1 messages where it had some extra room. The L8x will not do that and was therefore drowning from the excessive class 3 messages.
After fixing the WW, we tried again. All initially seemed good. However, once an hour or so, I would briefly lose my safety produce consume tags for a blimp - just enough to trigger an Estop - so I had to bail before running production. We ultimately got an AB engineer into help. He found that some of the ENET cards were not set up as unicast and we didn't have gateway addresses specified in all the ENET cards. Again, the L72S NEVER showed any issues with this, but the L82S did. Once he fixed those things, the comms were solid and we thought we were good to go.
When production started up, everything seemed to be working well for a few minutes and then we had an error on a MAOC instruction (OT-if you motion types don't know what that is, look it up - pretty cool). Anyway, we couldn't figure out what was causing the issue, so we aborted back to the L72S again.
After talking with AB, they are recommending that we read the entire 158 page document very carefully and then go through the program in slow detail to find all the things that could be affected. We are contemplating having AB do this ($$$$). Hoping we can learn from them and do the next one ourselves.
I don't know how much this helps you, but be warned that it may not be as straightforward as you initially thought. Read the document!
As some guesses. Your ENBT may not be set up right or may not be sufficient. You may need to go to a newer revision or to an EN2T. Don't assume that because your setup worked with the L55 that it is right and will work with the L8.
If you are using any produce consumes, READ THE DOCUMENT. There are some distinct differences in how an L8x handles them versus previous generations. I don't recall MSG's, but they may also be different. READ