The blocks being rejected by 0.7 are:
1. Possible to generate with 0.7 as well as 0.8
2. Perfectly valid blocks according to standard.
3. Being rejected because of a bug in 0.7
It will never, ever matter which blocks can be generated in regard to forking.
What matters is what blocks are accepted and what blocks are rejected. If this is consistent, no forks will happen. If this is inconsistent, a block accepted by one but rejected by the other is very likely to cause a fork if the ones that accept have the majority of hashing power. The ones that reject will then be permanently split off.
With bank accounts, consistency in transactions is far more important than avoiding minor bugs in implementing the specifications that don't affect the consistency or correctness of the transactions. This is why they made the decision to switch the mining pools to 0.7 for now. This gives the vast majority of hashing power to a universally-consistent chain.
They can set a date and time for everyone to switch to accepting 0.7-incompatible blocks. Then we'll again be consistent (as far as this problem), though people will be out of luck if a fork happens, until they upgrade.
But the thing is they'd set a date to switch over what block-producers will
accept, not what they produce.