Well, the 994 is no longer supported by UD. If UD assistance is required for a problem, you will need to pay for it. There will be no further development work done for it, so no firmware updates or new features. Currently, there are some new devices that will not work with the 994.
If your existing unit fails and needs to be replaced, how inconvenient will it be to not have your automation working for a week or two? (I realize you said you have a spare ISY, but do you know it works?)
From my perspective, I would rather plan a replacement/upgrade rather than have to do it on an emergency basis.
Also, UD is offering a 15% discount right now.
As for needing to update programs, including Elk, yes there will be some of that. However, if you copy all programs to clipboard and then paste to a document file, you will have a good reference to simplify the task.
Are you still using the old Elk Module, or are you using the Elk plugin? At some point, UD might be shutting down the Elk Module since support has ended.
If you are not using any plugins, a lot of new features have been added via plugins.
Once again, I would prefer to do this as a planned activity.
Only you can answer what is best for you.