Converting MBOX files to EML in bulk saves a lot of time when several mailboxes need to be handled together. Rather than opening each MBOX file separately, select the required files in one session, choose EML for the output and set a folder where all converted messages can be kept. This approach is particularly useful when MBOX files have come from different mail applications such as Thunderbird, Apple Mail or Entourage.

A desktop converter such as DataVare MBOX to EML Converter Expert can handle multiple MBOX files together and save the resulting EML data locally. It also lets you select only the MBOX files you actually need to convert.

I would run a small batch first, check a few EML messages and attachments and then process the remaining mailboxes.