Soft fork by definition doesn't require any upgrade. Miners dont need to use that. Soft forks means you follow tighter rules than the normal consensus rules. For example if the network is 1 MB, but you create 300 kB blocks, that'd be a soft fork.
Hard fork is widening the rules, 2 MB... because it breaks the consensus of 1 MB and if someone doesn't follow, it results in the split.
Soft fork by definition doesn't require any upgrade. Miners dont need to use that. Soft forks means you follow tighter rules than the normal consensus rules. For example if the network is 1 MB, but you create 300 kB blocks, that'd be a soft fork.
Hard fork is widening the rules, 2 MB... because it breaks the consensus of 1 MB and if someone doesn't follow, it results in the split.