Convert pfx to PEM:
openssl pkcs12 -in certificatename.pfx -out certificatename.pem
Do this dumps out a single plain text file.
Now how do I convert this plain text pem back to pfx?
The only commands I see to convert to pfx require the cer and private keys in separate files:
Convert CER and Private Key to PFX:
openssl pkcs12 -export -in certificatename.cer -inkey privateKey.key -out certificatename.pfx -certfile cacert.cer
Although I concur this is not really programming and arguably offtopic, numerous similar Qs about commandline tools (openssl, keytool, certutil etc) for (crypto) keys and certs are apparently accepted by the community (upvoted) -- but none I've seen directly addresses this point.
The different options on openssl pkcs12 -export allow you to provide the pieces in different files, but that is not required. If you do have the privatekey and chain of certs in one PEM file, as output by default by pkcs12 [not -export], you can let everything be read from that one file:
openssl pkcs12 -export -in file -out p12
# or ONLY IF the privatekey is first in the file
openssl pkcs12 -export <file -out p12
and you can even combine the pieces 'on the fly' as long as you put privatekey first:
cat privkey.pem mycert.pem chain.pem | openssl pkcs12 -export -out p12