Although CARTO is currently encouraging older users to migrate to the new UI, Builder, the change hasn't been enforced yet. If you wish to switch from Editor to Builder all you have to do is send us an email to email@example.com and we'll take care of that for you.
Before switching to Builder, there are some things you may want to note:
1) After the migration, user's can't be switched back to the older CARTO Editor. This is due to the fact that Editor is currently being deprecated, and technical support, bug fixes, etc; won't be available any more.
2) So many things have changed between CARTO Editor and Builder that many features are not compatible. Although many changes have been taken towards ensuring that older Editor maps won't break when opened with Builder, its still not recommended to do so. Some features, such as logo's or text over the map, are not available in Builder and can't be edited in the new UI.
That said, there are a ton of new features available in Builder, which you can learn about by checking our CARTO Guides.
Note: I'm currently working in CARTO's Support Team