I wouldn't change the timestamp to a string, otherwise it's no longer a valid timestamp, just on principle. Plenty of functions to translate timestamps to something human readable. If you just intend to store the same integer as a string representation of that integer, that may be OK, the usual typing mechanisms should be able to work with that. Or add something like a UTC_string field if you really need a string without doing any conversion.
The nature of meta data is it can be expanded and contracted based on what's available, so I don't think there would be an issue with doing so. WP seems pretty good at initializing defaults for any data it needs to use if it does not exist already. However, I don't know for sure if there will be problems, it's just my gut feeling there won't.