Is there a word for re-engaging in a habit one used to have?

"resume a habit, a hobby, etc"

• resume - (verb) - take up again after interruption.

• "Soon after the holidays, the children were back in school, and I resumed my hobby of sewing."

• "In England, about one-third of primiparous women resumed intercourse by 6 weeks."

• "The Christian" was first published in November 19025 and discontinued in 1928. The magazine resumed publication in July 1934."
• "About noon we resumed our journey, and soon after five P. M. we reached Kaau."

If it is an undesirable hobby, such as excess consumption of alcoholic beverages, "backsliding" is sometimes used. Somewhat more common is the phrase "fell off the wagon".

"I used to play piano daily a long time ago and plan to take it up again soon."

One takes up a hobby. "I took up snorkeling when visiting the reef."

Macmillan Dictionary:

to start doing something regularly as a habit, job, or interest

Revert to type

revert to type Popularity: Bottom 10% of words Definition of revert to type chiefly British : to go back to doing what one usually does

Also if the old habit was considered a bad one the term relapse is appropriate.

In the case of renewing interest in something that used to be a passion,like playing piano, one might say: "He plans to rekindle his passion of piano playing."

Or as often is expressed by musicians who have taken a hiatus from their instrument: "I plan to pick up ( instrument name) again."