Is there any way to convert an (LTC) M prefix address to an L prefix address?
Many services an apps that accept and transfer LTC reject M-addresses as invalid, even though they are given out by Coinbase.
I called Coinbase support but was unable to find a solution.
You cannot convert an M prefix to an L prefix as they are different types of addresses, the first being a P2SH address and the latter a P2PKH address.
You can, however, convert it to a 3 prefix address using https://litecoin-project.github.io/p2sh-convert/, a convertor built by the Litecoin Foundation for this exact purpose.
Originally, Litecoin used the 3 prefix for their P2SH addresses. However, this caused an overlap with Bitcoin, which also uses a 3 prefix, and led to many people sending funds to incorrect addreses. Litecoin then changed their prefix, but since it is simply an encoding change, you can convert it back to the original prefix for wallets that have not yet update their integration to respect the new prefix.
