A not uncommon style variation for theorem heads is to have the theorem number on the left, at the beginning of the heading, instead of on the right. As this variation is usually applied across the board regardless of individual theoremstyle changes, number-swapping is done by placing a swapnumbers command at the beginning of the list of newtheorem statements that should be affected. For example:
\swapnumbers
\theoremstyle{plain}
\newtheorem{thm}{Theorem}
\theoremstyle{remark}
\newtheorem{rem}{Remark}
After the above declarations, theorem and remark heads will be printed
in the form 1.4 Theorem., 9.1. Remark.