Historically, there were ida and idat to deal with 32 bits binaries. Then 64 bits binaries make an appearance to our world and this is how ida64 and ida64 were born.
Hex-rays decided to remove ida, mainly to simplify the product and the code base. It was done in two steps
make ida64 fully usable with 32 bits binaries
offer a tool to migrate dabase from idb (32 bits) to i64 (64 bits) format
Main usage
ida64 foo.idb
This command converts foo.idb to foo.i64 on the fly. The user can immediately start working with the database. If foo.i64 already exists, IDA will ask if it should be overwritten or not. Please note foo.idb is left untouched by the upgrade.
List of the available command line switches
--cvt64 automatically confirms the following question:
'You passed 32bit IDB file foo.idb to ida64.
Do you wish to convert it to 64bit database?'
-Ocvt64 options:
overwrite=yes/no/ask -yes: overwrite the existing i64 database
no: immediately exit if the i64 database exists
ask: ask the user what to do if the i64 database exists
default: ask
restart=yes/no/ask - yes: automatically open the converted database in IDA
no: exit after the conversion
ask: ask the user if the converted database should be opened
default: yes
privmove=yes/no/ask - move privrange beyond 32bit addrspace after
conversion?
default: yes, which means to automatically move
the privrange to 0xFF00000000000000
Example
idat64 --cvt64 -Ocvt64:overwrite=no:restart=no
This command automatically converts foo.idb and exits at the end of the conversion. If foo.i64 already exists, the conversion will not be performed. Since this command uses idat64, it can be used for batch processing.