If you have been using personal geodatabases stored in Microsoft Access, you might reach the maximum file size but still want to use a single-user geodatabase. You could move your data to a file geodatabase, which has a size limit of 1,000 GB per table. Or you might decide you want to take advantage of the ArcSDE functionality (versioned editing, history, and multigeneration replication) in an ArcSDE geodatabase for SQL Server Express but still want to use a single-user geodatabase. You can move the data in your personal or file geodatabase to an ArcSDE geodatabase licensed through an ArcInfo or ArcEditor license level of ArcGIS Desktop.
All these geodatabases are limited to single editors but can be accessed concurrently by a few users.
For all migrations, the easiest method is to copy the contents of one geodatabase and paste them into another.
NOTE: There are certain limitations between database types that could prove problematic if you move from a personal geodatabase to an ArcSDE geodatabase. These include spaces in the names of database objects, name length limitations, and data type conversion issues including memo fields (in personal geodatabases) being interpreted as binary large object (BLOB) fields.