Now the widest margin is on the right for odd pages and left for even pages. Is there a way to reverse/switch the left-right margins, so the widest margin is on the right for even pages and left on odd pages?
You can change the margins with the lengths \oddsidemargin and \evensidemargin, though using something like the geometry package might be more of what you're looking for. The right margin on each page is implied by the sum of the odd or even side margin and \textwidth.
As other comments have implied, make sure you actually need to do this and that it's a good idea before wading into the innards of the document class.
memoir also provides commands to modify the text area or the margins, aka the page layout. See chapter two in the memoir manual. But before doing so, you should indeed be sure, that there isn't another mistake or misunderstanding or misconception, because with xepersian I cannot reproduce your problem:
already produces a document with switched margins. Please note: The PDF always contains the pages in reading order not in printing order. So you have to print the document in reverse order.
Sorry, but I can no longer participate here as the administrator is trampling on my wishes on one of his other platforms.