The level of social mobility varied depending on a number of factors (period, region, ruler, person in question, etc.) but yes, there could be some social mobility - including someone being knighted for their service.

A great example of this is Thomas de Rokeby:

The young Edward III had just taken the reins of England from Mortimer and Isabella and had begun to wage war on the Scottish. After searching for the Scottish army in the border regions for some time, with no success, Edward promised a knighthood and land worth £100 a year to whoever led him to the Scottish army.

Thomas de Rokeby, a yeoman from Yorkshire, was among those who set out to find them. Thomas ended up being captured by the Scottish. He told them his mission and they thought it was funny, as they had been out looking for the English army with just as much success. They let him go back to Edward to complete his mission. He led Edward to the Scottish, received his knighthood and went on to become one of the foremost knights of the realm.