I am currently working on L1. I applied to a different company and they submitted a TN petition for me (i129 change of employer), which got approved. The start date is XYZ. and it isn't far. However I have still not received the hard copy of the approval notice (notice of action) from USCIS due to COVID19 delays. I cannot begin working at the new company until I have the original approval notice. While I wait for the approval notice, can I continue working on L1 at my current company beyond the date XYZ?