We have no reason to think a "mind" even exists at all!
Of course we do.
You and I evidently have different definitions of "reason."
It is called color and sound and pain and all these things that are transformations of one kind of information into something completely different.
You mean categories of different types of external stimuli that in turn trigger our capacity for associative problem solving metrics?
There is no reason for a brain that just experienced some stimulation to transform that stimulation into something else if the brain is the experiencer.
Category error. The stimulation triggers associations of previous like stimulations, which in turn combine with the current set of circumstances to form a unique new set of ALL (or near all) set of like circumstances/events/feelings/etc., that make up an "experience" for the brain, or, more accurately series of ongoing, always updating animation of experiences that all constantly flow and flux and update and degrade in a whirl of process identical in form to an animated film that creates the illusion of progression/continuity, because that continuity was found to assist us in survival.
If the brain is an experiencer
It is.
it does not need to change the experience it just had into a completely different kind of experience.
It doesn't, unless you are under the impression that a stimulus in a skin cell or nerve in the leg is the total sum of any such "experience."
There would be no reason to do it.
See? We clearly have different definitions of the term.
If a brain transforms an already had experience
It doesn't, necessarily. The process of triggering is evidently what results in an initial stimulus packet of information being associated with other like stimulus packages (and their ancillary associations as well) being added to along the way until at some nexus point this new stimulus package is packed away as no longer new and is replaced by another different stimulus package, ALL of which in the hunt for red october (survival; or, better, optimally beneficial outcome). Add, rinse, repeat until death do you part.
it is reasonable to conclude it is doing it because the brain is not experiencing the original experience, what is called stimulation.
And now that this is in its proper context, I agree. It is reasonable to conclude that by the time any external stimulus relayed by the body to the higher processing centers of the body--aka, the "brain" in a collective sense, but not necessarily limited to just that which is in our skulls--would have additional information added to the original stimulus package (as well as some information removed from it) before it reached an apex of information processing--that we refer to as an "experience"--and it gets replaced by the next package.
ETA: Though, technically, the "apex" of any given stimulus package (aka, "experience") is something that we refer to
just after the apex (ie., in retrospect), once the totality of the circumstances that make up the experience have passed. For example, a car crash. Usually the "experience" of a car crash comes just after the collected stimulus (and all the associations they trigger) of the car crashing.
So, "experience" is really more of a cataloguing process of stimulus/information/associations that arose at a given point and due to specific circumstances and only just after it has all been processed, but I suppose that all depends on the nature of the experience (aka, stimulus trigger/association package).