Standard Base Calendar update

Hello,

I´d like your advice. I have the following situation:

I have a CA PPM client that has projects planned last year (2016), these projects have tasks scheduled to perform during 2017-2018-2019. When these projects where planned, the standard calendar did not have defined yet the 2017-2018-2019 non- working days, so tasks duration for tasks executing during this period does not consider these years non- working days. The calendar was updated on Nov 2016 with 2017 non-working days,  but tasks remain with their original start-finish dates and duration. Client thinks CA PPM should update automatically tasks duration each time calendar changes.

Now, They need the wrong tasks duration be adjusted, but there are a lot of task to do it manually.

My question is: Is there a way to perform a batch process in order to recalculate an update wrong tasks duration?. What would you recommend?. 

What best practice should be applied to manage situations where you plan a project that will last for example 3 years and when the project is  planned not necessarily all the non-working days are known in advance.

Thanks very much for your help.

Best regards

Pilar