I think it is important to state that there are two types of ways for modifying the rules for every cryptocurrency blockchain: soft fork and hard fork.
A soft fork makes the blockchain generation rules more strict, so it is backwards compatible. It can't create a split chain (in general)
A hard fork makes the blockchain generation rules less strict. That means that when the new rules are active, those that have not updated will be in a different chain that.
The Bitcoin Core upgrade suggestion can be applied using a soft fork. On the contrary , the blocksize increase requires a hard fork.
The Bitcoin Core upgrade is already working. Bitcoin Cash was created because a group of miners leaded by ViaBTC rejected that soft fork and made a hard fork to counter it. Only users that decided to install the new software created by Bitcoin Cash moved to that split, those that have older versions remained on the Bitcoin chain.