For better, for worse
Does defense matter in fantasy baseball? While it doesn't factor directly in scoring, it does matter where your players appear in the field. Players start the season with position eligibility based on the prior season. Players that switch positions have to play a minimum number of games before they meet their league requirements for eligibility at the new position. There are two time lags: one in retaining legacy positions and another in qualifying for new positions.
Switching from the outfield to a middle infield position can greatly enhance fantasy value. Middling production from an outfielder equates to above average production at second base, for example. Players that recently gained eligibility as a middle infielder or catcher or that are on the cusp of doing so are for better.
Position changes are not necessarily beneficial, however. There are a number of position players that are effectively trading down. They are moving from shallow fantasy positions to deeper fantasy positions. They will keep that legacy position for the remainder of 2009 but lose it next season. Enjoy the eligibility while you can since their long-term values will reduce (for worse).