Sadly, I don't know how some of these changes will affect me.
I don't use the bundled web client, so I will ignore those changes.
When your instance admin, or more likely their hosting company, upgrades the software to the newly released version, then you'll discover if something broke in your client.
Adding two API versions to Mastodon would be great! Then your client can switch to the -previous API until it's updated. App devs could use -next to prepare.