Ah, forgot about those. They're safe to delete, except for the latest log file. I can't remember what the process is that creates a new sequence of files. Suggest deleting any log files more than 24 hours old, it's pretty easy to do with PowerShell.
If the log files are large (>50MB or more than 100K lines) you might want to analyze them and see if you're getting an excessive number of certain errors, that could point out a problem or bad configuration. (For instance, I had multiple errors when SSRS tried to initialize Oracle and Teradata data providers, which weren't installed. Commenting those providers out in the config file solved that).
I'd suggest doing that analysis before doing any kind of migration or move to a new SSRS instance, since doing so might not solve anything.