Q. Can a person switch from week to week sometimes going with plag of G’ra and sometimes Magen Avraham?

A. Establishing a day in Halacha requires dividing the day into twelve equal parts also known as Sho’os Z’maniyos. There are different opinions from when we begin and end counting that time exactly.

The opinion of Hag’ra and the Ba’al HaTanya, we measure from the time from sunrise to sunset, and then divide this period into twelve parts.

The second opinion is the Mogen Avrohom and the counting is done from the time of dawn (thr very first light on the eastern horizon), until the seeing the stars and that period is divided into twelve equal parts.

This view can only work according to the shitta of Rabenu Tam and other Rishonim regarding the onset of nightfall or Tzeis Hakohovim.
Therefore, in practice regarding Shabbos it is difficult to keep the Plag of the Magen Avrohom.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a