Could an old third rate become a transport?

Could third-rate ships of the line be used as freighters later in their careers?

Yes. Many were armed en flûte and used to move troops or stores after they were no longer needed in the battle line.

Conversion was a normal way to extend the utility of a large wooden hull. Removing guns reduced manpower and freed deck space for soldiers or cargo. The resulting ship retained naval ownership and structure without serving as a full battle-line unit. This reverse conversion complements the examples of East Indiamen rebuilt in the opposite direction as fourth rates.