multiple plans?

1 - PREP.  Get a calendar and write down the dates of the Saturdays you will be switching.
2 - Import the marathon plan into a future date (2007) that won't interfere or overwrite current (2006) planned data.
3 - Go into your 'Planned Training' blog-view, then use the 'copy/move/delete' link for each long marathon run to 'move' or 'copy' into the 2006 Saturday dates.  Use the minicalendar forward/back to move throught the months. 
4 - Confirm when all done and bulk delete 2007 data.

There are other ways to do it like when 'importing' you can select only a single day (example, day 46) and import into a 'one-day' range.  You will have to know the days of the marathon plan and dates of your Saturdays in advance.

http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=480 and

http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=416