As your ink passes the shelf life, you will typically see an increase in viscosity. The thicker material becomes more challenging to print, and may require a larger kick value. With the increased viscosity, you may also see the electrical properties and solderability suffer, as the higher viscosity material may not cure completely.
However, if the ink continues to work for you, the shelf life is a recommendation based on our testing, not a hard stop. If you want best results, fresh ink will work best, but you can use expired ink as long as you are ok with the risk that print quality and performance may suffer.