In the name of Allah the Most Merciful
Yes, it is permissible to read the Surah upto where one knows. (Shami vol 1, Al-Hidayaah Vol 1)
“Recite that from the Quran which is easy for you” (Surah Muzammil, Verse 20)
In esteem of the Ayah above it is good that one reads Surah in the Salaah which one has memorized and will find comfortable to read, as this will help revise the Surah and one will be able to learn more thereafter, Inshallah.
Lastly, it is important to know that the order in which one recites the Surah’s of the Quran in Salaah is important. I.e. the order in which the Surah’s are presented in the Quran must be adhered to.
And Allah Taa’la knows best
7 September 2005
Mufti Abubakr Karolia
Founder of the “Islamic Foundation for Theology and Research” (I.F.T.A.R)