My gene has changed, and I don't know why.
The Ensembl gene set is updated when a new genebuild is performed. New evidence such as recently submitted cDNA and protein sequences, and updated transcripts from the VEGA/Havana project (for human, mouse, and zebrafish) are included in updated gene sets. A gene may look different in a more current Ensembl version if new evidence has been included, and if the underlying genome sequence has changed.
Genome sequences, or assemblies, are changed when new evidence allows gaps to be filled, or errors to be corrected. A genome assembly is usually updated once a year or less often. The stop and start coordinates of a gene may change if there has been an update in the underlying sequence assembly.
The news for every Ensembl release (or update) explains the changes within the new version (such as new genebuilds and new assemblies). To find out what assembly you are working with in Ensembl, go to any species home page.