Can we use old Letters of recommendation?

According to ERAS,

If you participated in the previous ERAS season, you are not required to resubmit any LoR that was transmitted to the ERAS PostOffice for that season. Note that this only applies to LoRs that were transmitted to the ERAS PostOffice in the previous season. If you submitted an LoR but did not reserve a slot for the LoR in MyERAS , that LoR was not transmitted to the ERAS PostOffice and will not be available for reuse for ERAS current season. Information for repeat applicants can be found here, for repeat applicants

If the content of your LoR has changed, submit the modified LoR. 

If you submit a modified LoR, it will permanently replace the previously uploaded LoR. 

All documents mailed to ERAS Support Services for scanning should be accompanied by a completed document submission form