Recently Sony updated the PlayStation Plus FAQ stating that they're disabling PS Plus subscription stacking via pre-paid voucher code cards for existing customers temporarily until after the launch of the upcoming PlayStation Plus & PS Now Service later this year in June. 
This comes proceeding the 10 Million Dollar XBox Gift Card Cheat, PlayStation Store Payment Option Limitations announced last fall and the Incoming PS4 / PS5 PS Plus Games next month.
Below are some relevant excerpts from their recently revised PS Plus FAQ:
I am an existing PlayStation Plus and/or PlayStation Now member. Why do I get an error message when I try to add time to my membership either by purchasing more time on the PlayStation Store on console/web/app, or by applying a membership voucher?
As we prepare to launch the new PlayStation Plus membership service, we are doing some work behind the scenes to make the transition as smooth as possible for all of our existing members. As part of this work, we’ve temporarily disabled stacking memberships for existing customers until after the launch.
Rest assured that your voucher code is still valid, and you will be able to redeem your code either when your existing membership expires and deactivates, or after the new PlayStation Plus service launches in your area, whichever happens first.
I have a PlayStation Plus or PlayStation Now 1-month, 3-month or 12-month membership voucher. What will happen if I redeem it before the new PlayStation Plus service launches?
If you currently have a PlayStation Plus or PlayStation Now membership, due to changes we are making to the service prior to launch, you won’t be able to redeem a voucher code for that service until your existing membership expires and deactivates, or after the new PlayStation Plus service launches in your area, whichever happens first. Rest assured that your voucher code is still valid.
Finally, PushSquare.com also notes the following on this discovery to quote:
"The website includes a link to a conversion chart but, as of the time of writing, that points to a blank page."
This comes proceeding the 10 Million Dollar XBox Gift Card Cheat, PlayStation Store Payment Option Limitations announced last fall and the Incoming PS4 / PS5 PS Plus Games next month.
Below are some relevant excerpts from their recently revised PS Plus FAQ:
I am an existing PlayStation Plus and/or PlayStation Now member. Why do I get an error message when I try to add time to my membership either by purchasing more time on the PlayStation Store on console/web/app, or by applying a membership voucher?
As we prepare to launch the new PlayStation Plus membership service, we are doing some work behind the scenes to make the transition as smooth as possible for all of our existing members. As part of this work, we’ve temporarily disabled stacking memberships for existing customers until after the launch.
Rest assured that your voucher code is still valid, and you will be able to redeem your code either when your existing membership expires and deactivates, or after the new PlayStation Plus service launches in your area, whichever happens first.
I have a PlayStation Plus or PlayStation Now 1-month, 3-month or 12-month membership voucher. What will happen if I redeem it before the new PlayStation Plus service launches?
If you currently have a PlayStation Plus or PlayStation Now membership, due to changes we are making to the service prior to launch, you won’t be able to redeem a voucher code for that service until your existing membership expires and deactivates, or after the new PlayStation Plus service launches in your area, whichever happens first. Rest assured that your voucher code is still valid.
Finally, PushSquare.com also notes the following on this discovery to quote:
"The website includes a link to a conversion chart but, as of the time of writing, that points to a blank page."
