Yes, it's possible, but you'll want to indicate in your explanation on the Form 1040-X that either you simply made an error when originally reporting the amount that was contributed to the SEP IRA for 2019, or, if the March SEP contribution was made after you filed your 2019 tax return, you can indicate that the SEP contribution for 2019 was increased before the filing deadline but after you filed your original tax return.
This is possible because it's the responsibility of the employer (in this case, you as self-employed), not the SEP-IRA custodian to track the year for which the SEP contribution is made. The SEP-IRA custodian only tracks and reports the year in which a SEP contribution is made.
