Here are the steps I came up with for converting old DTEXT to MTEXT. Select Similar is your friend…
Select all text your DTEXT. You can use select similar.
Select Similar, Make all your DTEXT the same LEFT justification, by using the right click JUSTIFY, this will keep its original location.
Select Similar, Use the t2mm.lisp to convert it to MTEXT
Select Similar, Change the justification to Middle Center with the right click JUSTIFY.
Select Similar, Under the Properties change the DEFINED WIDTH to “0” and turn on BACKGROUND MASKING
It seems pretty quick for me.
Good Luck.