When CPP issue T4A§ to someone first time for disability benefits and the approval letter explained that this amount is for 2018 and 2019. Should be report all amount in 2019 as reported on T4A§ or we can report it on T1Adj for 2018 and rest on 2019 as per the explanation on approval letter.
Generally: Report the amount of the 2019 T4A on the 2019 T1.
Report any amounts on a 2018 T4A on a 2018 T1.
Report any amounts on a 2017 T4A on a 2017 T1.
"Line 11400 – CPP or QPP benefits
If you have not received your T4A(P ) slip, go to canada.ca/esdc or call 1-800-277-9914.
Lump-sum benefits – If you received a lump-sum CPP or QPP payment in 2019, parts of which were for previous years, you have to report the whole payment on line 11400 of your return for 2019. However, if the total of the parts that relate to previous years is $300 or more, the CRA will calculate the tax payable on those parts as if you received them in those years only if the result is better for you. The CRA will tell you the results on your notice of assessment or reassessment.
CPP or QPP disability benefit (box 16)
Report on line 11410 of your return, your CPP or QPP disability benefits shown in box 16. This amount is already included in your income on line 11400 of your return, so do not add it again when you calculate your total income on line 15000 of your return. "
You can fill out Form T1198 though I understand that Service Canada provides the per year amount to CRA making the recalculation automatic.
If you adjust 2018 return, your client will be paying tax on the same income twice. CRA may or may not correct your error in the months to come.
In other words don’t touch the 2018 return.