How likely is it that the structure of a raw transaction will change? I've written a script that decodes raw transactions, as it's faster than calling the decoderawtransaction RPC.

How likely is it the structure of a raw transaction will change in the future?
All transactions people can reasonably create today will likely still be valid in the foreseeable future, the devs try really hard to stay as backwards compatible as possible.

But new features are introduced regularly. For example SegWit might require you to update your script. The recent Locktime BIPs may have already. SegWit also allows easier updates (or whole alternatives) of the scripting language, which will likely be put to use fairly quickly after SegWit rolls out.