Converting standard_conforming_strings from off to on when importing Postgres dump

I've got a Postgres 8.1 instance which has standard_conforming_strings set to off (== handles slashes as escape characters). Recent Postgres versions have this setting on by default (for good reasons). When importing a dump from 8.1 to 9.x, then I can set this setting to off in the 9.x DB but I don't want to, I'd prefer converting the dump into the proper format. This is a ~100GB dump. How could I do that? Can I simply turn this setting 'on' in the 8.1 version? (Will that affect exporting existing data? Will/can that corrupt or modify existing data?) What doesn't work: simply importing data produced by 8.1 to 9.x without setting standard_conforming_strings to off in the 9.x DB; the result is corrupted data.