The answer to this question says transactions with an unlock_time are mined right away but outputs will not be able to be used as inputs in another transaction until the unlock_time is met.
Based on the above would there be any danger in sending (or receiving) a transaction now with a 2 year unlock_time? Assume that planned hard forks continue every 6 months and that RingCT transactions are mandatory long before the 2 year unlock_time is reached.
There is no danger. The answer you quoted states that the transaction would be mined and confirmed like any other transaction. The sender can prove this with the same methods (payment IDs, etc) as any other transaction
The recipient would have the exact same ability to spend outputs from a 2 year unlock_time transaction as a recipient of a non unlock_time transaction today that left the received coins in cold storage for two years.
Just like no hardfork will harm the ability of anyone to spend coins stored in cold storage during multiple forks, no hardfork will harm the ability to spend funds from an unlock_time transaction once the unlock_time requirement has passed.
Measures will be taken to ensure there will always be a way spend funds in your wallet:
When someone uses a rct transaction, the outputs are necessarily rct. From hard fork 5 (expected in or around september 2017), non rct transasctions will be permitted only if they are spending unmixable outputs (ie, the rules for allowing mixin 0 now).
If you want to make all your outputs rct, you should be able to use sweep_unmixable first, then sweep_all. Assuming you do not have a large amount of tiny inputs, that should make your whole balance only in rct outputs.
