A Roth conversion from 2 years before Medicare, to anytime once you have begun receiving Medicare may or may not increase your part B premium. I have posted about this for several years. The magic number is a certain AGI as adjusted
figure, with tiers of Medicare premium boost depending on that adjusted AGI. The main adjustment is to add back non-taxable interest income to AGI. Thus, deductions on Schedule A do not help with this calculation.
Depending on your income before the conversion, and the size of your planned conversion you may be able to convert an amount yearly and still stay below the thresholds. My memory is that for a single taxpayer in 2009. the Adj. AGI threshold is $85,000. However, best to check this, as well as to check the threshold for 2010, if it has been announced, before you make plans.