Both sentences you give are grammatically correct, and I will describe the details below. I think perhaps you are not correctly remembering what you learned. In a general sense, much is nearly always used to qualify degree.
- For instance, when much is used to modify a noun, much describes an uncountable degree:
We use much with uncountable nouns and many with countable nouns
Many modifies things that can be counted (i.e., count nouns). Much
modifies things that can’t be counted (i.e., mass nouns).
- When much is used to modify a verb, or otherwise as an adverb, much is similarly considered an Adverb of Degree:
Adverbs of Degree in English help us to show the intensity and the
degree in which a specific action is done. These adverbs answer the
question: to what degree?
- Finally, when much is used to modify an adjective, much similarly expresses a positive degree of separation:
Where perhaps you became confused is that much is an irregular adjective in that while standard adjectives can take their base form to express positive degree, then add -er to take a comparative form and -est to express superlative degree; much is the base form but converts to more for comparative form and most for superlative form. You can review from these sites which you will note clearly show that much is the positive degree form:
Also, I note that in the second part of your question, you seem confused about the differences between much and more (and also much more which is the same as more except to an unquantifiable greater degree than regular more). Please note these differences between much and more:
So, understanding all the above, you can see that in your first part of your question where you use much different that much is appropriate and doesn't violate any rules - it shows a positive degree. In your second part of your question, by using more different you are implying that you are making a comparison between at least two things, and whether you say more different or much more different it still implies that comparison. The first part of your question (much different) doesn't imply any comparison between different things but only a degree of change in the same thing. So if you mean something like:
I had visited Washington D.C. ten years ago and returned for a visit
last week and my impression is that the city is not much different
now than it was ten years ago.
then use much different, but if you mean something like:
I had visited both Washington D.C. and Baltimore ten years ago and
returned to visit both last week and both had changed, but my
impression is that Washington D.C. is not much more different than
Baltimore, in terms of how much both have changed over the past
then use much more different