The value of the property must be over $1,000.00 to constitute a felony. Property stolen that is values between $501.00 and $1,000 is a gross misdemeanor. A gross misdemeanor is punishable by up to a year in the county jail and $3,000.00 fine or both. Any property stolen that is values at $500.00 or less is a misdemeanor. A misdemeanor carries a maximum penalty of 90 days in the county jail and a $1,000.00 fine. These are the general penalties but do not apply to every case. For instance, if the property is stolen from a corpse, is a certain controlled substance or is a motor vehicle. These are some examples of how certain theft offense becomes felonies regardless of the value of the property. In your case, the type of property alleged to be stolen might be determining the offense level rather than the value. You should have an attorney look at the complaint to determine why the charge is a felony. Finally, there could simply be a disagreement as to the value of the property. This would be a question of fact for a judge or even a jury to determine.
I believe that the question you are asking is when the limit was increased. Per the Minnesota Revisor of Statutes, the amendment raising the limits became effective August 1, 2009. So, a simple theft, of property with a value of $400, committed in September 2009 should not be charged as a felony.
It may be a simple mistake by the charging attorney, or there may be other factors, causing them to regard the matter as something other than a simple theft.
Most attorneys offer a free consultation, I would suggest taking the complaint with you and talking to a qualified defense attorney.