I'm currently trying to figure out how the new adult changes will affect users of older viewers like mine. Here are my findings so far, purely based on experimentation, so take them with a grain of salt. If you have other findings or better information, please post them in the comments here.
1) If you never logged in an avatar with a new V1.23 viewer, you seem to be able to teleport to the new adult continent without problem (e.g. go to the map, search for "Mosh" and just jump there).
2) If you have logged in with an V1.23 viewer at any time, the server seems to remember your setting from the General tab in preferences, even if you go back to an older viewer. So, when running/testing a V1.23 viewer make sure you choose "PG, Mature and Adult" in preferences, then click OK or Apply. After that you should be able to go to Adult parcels even if you switch back to an older viewer version.
3) I wasn't yet able to figure out how search is affected, because I could not find any parcels which appear in search and which are flagged as Adult yet. Does anyone know something which (when using the new V1.23 viewer) only appears when 'Adult' is checked in search? If so, I'd like to see how that appears in older versions.Update:
3) Search doesn't seem to be an issue with older viewers. In older versions, if you check the 'Include mature content' option in search, the viewer always also includes content flagged as 'Adult'. Being able to teleport there will then depend on the above (as described under 1. and 2.).
Bottom Line: Unless the Lindens change something, with old viewers like mine (or anything based on 1.22 like the current CoolViewers) you should be able to access all content if you are
a) using account with 'Payment info used' or 'Adult verified' status (this remains a basic prerequisite) and
b1) either you never logged in with an V1.23 viewer at all
b2) or you logged in with an V1.23, go to Preferences and set access to PG/Mature/Adult
Nick
PS: Experiment on this yourself with searching for a parcel/island called "Yasemine". It's a private island available in search and flagged 'Adult'