I think there should not be any problem changing to Hercules. I haven't done it yet myself, but it seems that on the SQL files you only need to run this on the ragnarok database:
---- Table structure for table `sql_updates`--CREATE TABLE IF NOT EXISTS `sql_updates` ( `timestamp` int(11) unsigned NOT NULL, `ignored` enum('Yes','No') NOT NULL DEFAULT 'No', PRIMARY KEY (`timestamp`)) ENGINE=MyISAM;-- Existent updates to enterINSERT INTO `sql_updates` (`timestamp`) VALUES (1360858500);INSERT INTO `sql_updates` (`timestamp`) VALUES (1360951560);INSERT INTO `sql_updates` (`timestamp`) VALUES (1362445531);INSERT INTO `sql_updates` (`timestamp`) VALUES (1362528000);INSERT INTO `sql_updates` (`timestamp`) VALUES (1362794218);INSERT INTO `sql_updates` (`timestamp`) VALUES (1364409316);---- Table differences for table `char`-- ALTER TABLE `char` CHANGE `moves` `slotchange` SMALLINT( 3 ) UNSIGNED NOT NULL DEFAULT '0'
And you now have Hercules databases.
For switching SVN I don't have a clue, but I think you should keep a backup of the last rAthena you used in case you want to undo all changes so I'd make a new folder for that.