Most tube upgrading services involve swapping out parts, primarily capacitors, for more expensive ones. This is not necessarily an improvement if the original designer voiced the component and used appropriate parts for the sound the designer was trying to achieve. I've heard several such "upgrades" that sounded clearly inferior (at least in my opinion based on my taste and the system the amp was tried).
To do this sort of thing correctly, one must find a technician who knows about the sound of different parts and you must convey what you want from the upgrade (e.g., warmer/lusher sound or greater clarity, etc.). There is no such thing as an always superior part.
Your best bet might be something non-invasive such as the tube rolling suggested above. Manufacturers put it the cheapest and most plentiful tubes that will fit their needs. They need tubes that can be easily replaced so that they can either supply the customer with replacements or make it easy and economical for the customer to replace. That is not just a matter of saving money. It does not make sense for a manufacturer to invest in expensive tubes of a particular type when that tube might not fit the customers' taste or match the customers' system. It makes more sense for the customer to do this.
