MT4: Can comments or magic number be modified if order open?

Want to modify comment (preferred) or magic after order is opened. Is this possible?

No.

The only things you can modify once an order is opened :

1/ TP/SL
2/ Lot size (partial closure)
3/ Entry price for pending order

I do not know why you want to change the magicnumber -> Can you tell us, maybe there is another way to achieve what you are looking for.
btw, trade filtering by ticketnumber is imho dangerous because brokers may change that number after a partial closure, ...

The safest way to filter trades is using magicnumbers and filtering by symbol.

I have EAs loaded onto several accounts. I also wanted to trade manually within those accounts or partially close manually trades already open. The EA assigns a magic and a comment to each trade. I was thinking... I could say close any trades where comment or magic equals zero, but if I had multiple open manual trades and wanted to run a script that only closed some of them, I wanted to be able to change the magic or comment to differentiate. As you say I could filter by trade number but it's a bit tedious if I had 20 open manual trades and wanted to close several of them via a script. I realize I could partially close any open positions that are larger than 0.01 size and assign a new comment, but what if I had 30 open positions of 0.01 instead of a single open position of 0.3 ?