Is yesterday just the past tense form of now? Or are they different lexemes entirely?
In English, only verbs have a past tense but the other parts of speech (nouns, adjectives, adverbs, ...) don't have one. The fact that you can express a notion of past-ness for some of them (e.g., former president or ex-wife) does not form a past tense.
Also now and yesterday aren't derivations from a common stem. While using common stems (or recognisably related stems) is not strictly necessary for a past tense (think of the past tense of the verb to be) it helps to have some morphological rules for forming a lot of past tense forms.
Together with the semantic arguments formulated by @curiousdannii there is no reason to think of now and yesterday as one lexeme.
Now can be analysed as a pro-preposition, a word standing in for a prepositional phrase. It isn't verbal, so the category of tense doesn't apply to it.
Now and yesterday are not parallel terms. Yesterday can refer to a whole day, but now doesn't even refer to a moment just before now. There's no reason to think that they're the same lexeme.