Will Electrum mnemonic seeds be forward compatible with all future versions of Electrum? If I use the newest version of Electrum (2.6.4) to create a mnemonic seed and the current source code (2.6.4) later becomes unavailable, will I be still able to use a mnemonic seed I create now with all future version of Electrum?

I know that Electrum has changed the type of mnemonic seeds it uses at least once (from 12 to 13 words). Should I be concerned that future updates will eliminate compatibility with current mnemonic seeds?

Are there any advantages of my retaining a copy of Electrum (2.6.4) if my only purpose in doing so is wallet recovery several years from now?
In fact, the reason Electrum 2.x changed to a 13-word seed* and is incompatible with BIP39 is because they added a version number to the end of the seed. This has the purpose of allowing future versions of Electrum update the seed format while still understanding how to derive all the addresses from previous seed versions.


  Standard Electrum seeds have 128 bits of entropy, plus 8 extra bits
  for checksum and seed version. Mycelium uses BIP39, which does not
  include a seed version.


https://github.com/spesmilo/electrum/issues/1733#issuecomment-200782966

Other clients rely on the community's formalisation practice through BIPs, where we all agree on a way to describe and encode/decode HD wallets. Years down the line, you should always be able to say "hey this was derived from BIP39" and everyone should still know what you mean. The developers of Electrum believe that including the version number and checksum is more important than adhering to a formal BIP format. There are no plans to propose a BIP for the Electrum convention.

"How do I make sure my cold storage wallet seed will be recoverable in x years?"

The standard answer is that Electrum is open source, and you should be able to find an archived version or at least find documentation on how to recover your wallet should the project shut down.

*It's not actually 13-words, it's just happens to have 13 words right now. In the future this may change to greater or fewer number of words, but it won't matter because the software will know the version/checksum.