How to change shareholders and structure

When i first set up my company 10 years ago , i had 49% ordinary shares and the rest of the 51% are held by Thais as preference shares.

Now i have a trusted Thai friend (let's say Mr A) whom i wish to have him hold 25% ordinary shares.

Is it possible to convert 25% from the 51% preference shares into ordinary shares? And how to transfer to Mr A?

Prior to this none of the preference shareholders singed any share transfer document.