No you cannot change how you want to receive your refund after you file your tax return. And....mailed returns piled up in trailers for several months during the pandemic. The IRS recently began to open over 11 million envelopes. So you will be waiting for quite awhile for the 2018 refund.
Per the IRS website IRS Operations During COVID-19: Mission-critical functions continue:
IRS operational status and alternatives
Processing Delays for Paper Tax Returns: Taxpayers should file electronically through their tax preparer, tax software provider, or IRS Free File.
We’re experiencing delays in processing paper tax returns due to limited staffing. If you already filed a paper return, we will process it in the order we received it.
Do not file a second tax return or contact the IRS about the status of your return.