Irrespective of the filed flight plan [a "plan" is not a clearance] with intended en route cruise level changes: It is is impractical reality for any airplane anywhere to change its last assigned Flight Level without talking to somebody.
Take this example: One day last year we were en route SIN-DXB. We were "cleared as filed" and over the Arabian Sea our "filed" flight plan had directed us to climb from FL340 to FL360. But Mumbai HF radio was dead on all frequencies. No other airplane over the Arabian Sea was able to contact Mumbai Radio. Needless to say, we maintained our last assigned Flight Level [thankfully as did all the other airplanes] until we had established VHF contact with Muscat Center. Between waypoints SUGID [Mumbai FIR] and PARAR [Muscat FIR], the radio outage lasted approximately 1 hour. It would have been suicidal for us to have followed our flight plan and to have climbed on our own in this busy airspace.