Can we perform maintenance and main updates in 2017 and 2018 for our customers as of the year-end, even if the system does not comply with the requirements in the Cash Register System Act and the Cash Register System Regulation?

Yes, you can continue to perform updates to cash register systems for customers that you have as of 1 January 2017 until 1 January 2019 without breaching the cash register system regulations. After 1 January 2019, all those who are required to keep accounts must use a cash register system with a product declaration. 
If you wish to sell, lease out or lend cash register systems to new customers after 1 January 2017, the cash register system must satisfy the cash register system regulations.