If people insist I can post a copy of the entire PEP here on the list, but since a lot of it is just background information on the old LL(1) and the new PEG parsing algorithms, I figure I'd spare everyone the need of reading through that. Below is a copy of the most relevant section from the PEP. I'd also like to point out the section on performance (which you can find through the above link) -- basically performance is on a par with that of the old parser.
This section describes the migration plan when porting to the new PEG-based parser
if this PEP is accepted. The migration will be executed in a series of steps that allow
initially to fallback to the previous parser if needed:
1. Before Python 3.9 beta 1, include the new PEG-based parser machinery in CPython
with a command-line flag and environment variable that allows switching between
the new and the old parsers together with explicit APIs that allow invoking the
new and the old parsers independently. At this step, all Python APIs like ``ast.parse``
and ``compile`` will use the parser set by the flags or the environment variable and
the default parser will be the current parser.
2. After Python 3.9 Beta 1 the default parser will be the new parser.
3. Between Python 3.9 and Python 3.10, the old parser and related code (like the
"parser" module) will be kept until a new Python release happens (Python 3.10). In
the meanwhile and until the old parser is removed, **no new Python Grammar
addition will be added that requires the peg parser**. This means that the grammar
will be kept LL(1) until the old parser is removed.
4. In Python 3.10, remove the old parser, the command-line flag, the environment
variable and the "parser" module and related code.