How to switch a leaded solder soldering station over to lead free solder
In an ordinary engineering development situation of working on prototypes or other non-production items not needing to meet stringent material or longevity standards, switching solder types would (if thought about at all) typically be done by a few extra cycles of tinning and wiping the tip.  As Leon points out, adjusting the iron temperature may also be needed.

If you are facing a requirement which a few cycles of such re-tinning and wiping cannot satisfy, then you need a new tip, if not an entirely new setup and workspace.

In functional terms if the decision is made to still use leaded solder at an engineering bench in an non-exempted industry where lead solder would not be permitted for production, then there's likely to be a mixing of alloys on an effectively continuous basis, since there's a reasonable chance that lead free solder was used in the initial assembly of any machine-built prototypes, and a near certainty that it was specified to be used on any production items that find their way back into the lab for failure analysis, as a basis for prototyping new ideas or versions, etc.  In most cases, once something has been open on an engineering bench, it shouldn't be thought of as "product" any more anyway.