How do I know in which new HSK level should I enroll, based on the old HSK levels?

It's advisable to review the specific vocabulary, grammar, and language skills required for each level of the new HSK exams to determine the most appropriate level for your current proficiency.
Old HSK Level 1: Equivalent New HSK Level: New HSK 1 or 2
Old HSK Level 2: Equivalent New HSK Level: New HSK 2 or 3
Old HSK Level 3: Equivalent New HSK Level: New HSK 3 or 4
Old HSK Level 4: Equivalent New HSK Level: New HSK 4 or 5
Old HSK Level 5: Equivalent New HSK Level: New HSK 5 or 6
Old HSK Level 6: Equivalent New HSK Level: New HSK 6

Based on numbers of characters:

New HSK 1 ~ Old HSK 3
New HSK 2 ~ Old HSK 4
New HSK 5 ~ Old HSK 5
New HSK 6 ~ Old HSK 6

You can check the number of words you should know for new HSK levels and compare it with previous HSK levels.

The content of the material is roughly the same. if you really want to know whether it meets the current level, you can know it through the new material content test.