Question

For the stem-changing verbs in the present tense

I wouldn't have to do anything to those verbs in the preterite tense except changing the endings?

Answer 1

so o -> ue verbs would be left only in the preterite tense?

Answer 2

Actually the Preterit is the most difficult of all the tenses. There are regular verbs, -ar verbs using one set of endings and -er/-ir verbs using a different set of endings.

Verbs ending in -car, -gar, and -zar have a spelling change in the stem in the fist person singular: c = qu / g = gu / z = c

*Stem-changing verbs ending in -ir that have a stem change in the Present Tense also have a stem change in the Preterit. This change occurs in the 3rd person singular and plural, from e to i or from o to u.
servir = serví, serviste, sirvió, servimos, servisteis, sirvieron

Then there are many irregular verbs.

I hope that covers what you needed, because I do not understand your question.

Sra