SQLite converters are documented to be sensitive to the case of type names, but they're not.

In [50]: import sqlite3
    ...: 
    ...: sqlite3.converters.clear()
    ...: sqlite3.register_converter('T', lambda x: 'UPPER')
    ...: sqlite3.register_converter('t', lambda x: 'lower')
    ...: 
    ...: con = sqlite3.connect(':memory:', detect_types=sqlite3.PARSE_DECLTYPES)
    ...: cur = con.cursor()
    ...: cur.execute('create table test(upper T, lower t)')
    ...: cur.execute('insert into test values (?, ?)', ('X', 'x'))
    ...: cur.execute('select * from test')
    ...: cur.fetchone()
    ...: 
Out[50]: ('lower', 'lower')

In [51]: sqlite3.converters
Out[51]: {'T': <function __main__.<lambda>>}

Original commit in pysqlite that makes converters case insensitive: https://github.com/ghaering/pysqlite/commit/1e8bd36be93b7d7425910642b72e4152c77b0dfd

It's worth noting that the referenced commit where this change was made is from 2006...

New changeset 831c29721dcb1b768c6315a4b8a4059c4c97ee8b by Tal Einat (Sergey Fedoseev) in branch 'master':
bpo-34018: Doc'd that type names of SQLite converters are case-insensitive. (GH-8042)
https://github.com/python/cpython/commit/831c29721dcb1b768c6315a4b8a4059c4c97ee8b

I tested and indeed this is case-insensitive even on 2.7.

New changeset 6699386231d326c50439b9e1df752e1aed7ca6b4 by Tal Einat (Miss Islington (bot)) in branch '3.7':
bpo-34018: Doc'd that type names of SQLite converters are case-insensitive. (GH-8042) (GH-8065)
https://github.com/python/cpython/commit/6699386231d326c50439b9e1df752e1aed7ca6b4

New changeset f55a818954212e8e6c97e3d66cf1478120a3220f by Tal Einat (Miss Islington (bot)) in branch '3.6':
bpo-34018: Doc'd that type names of SQLite converters are case-insensitive. (GH-8042) (GH-8067)
https://github.com/python/cpython/commit/f55a818954212e8e6c97e3d66cf1478120a3220f

New changeset 292ce153fcc2d991164d19ad3f3deb86157898d5 by Tal Einat (Miss Islington (bot)) in branch '2.7':
bpo-34018: Doc'd that type names of SQLite converters are case-insensitive. (GH-8042) (GH-8066)
https://github.com/python/cpython/commit/292ce153fcc2d991164d19ad3f3deb86157898d5