Früher lernte/habe... ich am Morgen Deutsch/morgens Deutsch ...gelernt und Nnachmittags studierte/habe... ich die Universität-Fsfächer ...studiert.

Präteritum is ok to use but it seems a bit too formal for what appears to be a somewhat casual report.
'Am Morgen' is correct but, again, it seems overly formal and with 'morgens' you're better in line with 'nachmittags'.
Then, the Fugen-S in 'Universitätsfächer' is tough to explain, but it needs to be there. Quite frankly, a Fugen-S will have to appear wherever native speakers feel like it, because it would "sound wrong" without.

Allerdings werde ich ab morgen früh Mmorgens Universität-Fsfächer studieren und lerne ich N(ich werde) nachmittags Deutsch lernen.

It is very elegant if you have the part after the 'und' still be dependant of the 'werde ich...' at the start of the sentence, but you could repeat the 'ich werde' too. As you had it, it was quite a mess, with the verb being in the first position (instead of its very own, reserved second position) and a different tense than in the first half of the sentence being used.