Event canceled and want to change the registration fee into a donation

I had a similar situation which required converting tickets into donations. I did this by going through the process of refunding the ticket to the constituent and in the bottom area where it says "Refund Method" under the "Payment Details" I set it to "Other" and in the Reason area I put "Exchange" and in the details area, I simply described that the ticket was going to be converted into a donation. Once I submitted the refund, I had to process a new transaction and went on to the daily sales tab, searched the patron, added the donation (which was the total amount of the ticket purchased) and for the payment information I set it to the "Exchange" option and completed the donation. 


Here's what I followed:
https://kb.blackbaud.com/articles/Article/103485


Also there was no "Exchange" option in the payment method before, so I created a custom payment.

Creating a new payment method
In Administration, click Code Tables
In the Revenue section, click 'Other Payment Method Type'
Click Add, enter the name (Ex. Exchange), click Save


Also I would suggest creating a report with the list of people converting their tickets into donations and the amount, just to keep track.

I hope this helps!