Short answer: it’s not trivial.
Fortunately, the developers anticipated this problem and deal with it in an FAQ:
http://subversion.apache.org/faq.html#working-copy-format-change
The upshot being to download and use their script for the purpose:
http://svn.apache.org/repos/asf/subversion/trunk/tools/client-side/change-svn-wc-format.py
Note that that script only wants the major version number of the client, so if you have client version 1.4.4 the command would be:
python change-svn-wc-format.py <WC_PATH> 1.4 [...other options...]
Update:
The above script only works for downgrading version 1.6 and below. Downgrading from 1.7+ is apparently not possible. The note from the source:
# Downgrading from format 11 (1.7) to format 10 (1.6) is not possible,
# because 11 does not use has-props and cachable-props (but 10 does).
# Naively downgrading in that situation causes properties to disappear
# from the wc.
#
# Downgrading from the 1.7 SQLite-based format to format 10 is not
# implemented.