I have a business class ticket for my flight with Lufthansa from Venice and I have a connecting flight from Frankfurt. Then my other flight is in Economy class with Sunexpress. Can I still access to Lufthansa lounge in Frankfurt Airport using the (old) business boarding pass even after I arrived there? Or I can't access there because my next departure is not Business class anymore?
Lufthansa Rules are notoriously complicated and at the moment there doesn't seem to be anything published officially. At least I couldn't find it.
A recent trip report that was a similar case was denied access, since lounge access through class of service is determined by the departing ticket, not the arriving (or the whole itinerary).
If this is on a single ticket you can certainly walk up to the counter and ask. See what happens. If these are two separate tickets, the answer is no.