While I can't say, for sure,that a league-convert's running ability will go backwards, I will say, with a high degree of certainty, that it will not improve. This is especially true for a guy like Wendell Sailor. First of all, union wingers/players don't get anywhere near as involved as their league counterparts. The reasons for this are -
a) In union, the ball is in play for less than half the time that it is in league, so this reduces the amount of opportunities to be involved.
b) It is very common for a league winger to leave his flank and become involved by running the ball. Many league wingers have an extremely high involvement rate due to this. In union, it is much more important for a winger to stay on his wing, as if there is a turnover in possession, an overlap can very easily open up, due to the fewer amount of players in a union defensive line.
c) Whenever a league back gets the ball in his own in-goal area, or in his own quarter, he will always run it back - it is one of the things that a league fullback or winger prides himself on. In union, the ball is almost never run out of the in-goal area - it is usually forced for a 22-metre drop-kick. Also, in union, when the ball is retrieved inside the 22-metre area, most of the time it is kicked out.
The second reason why a league player would not improve his running abilities inunion is because the quality of the runs are not as good. This is because -
a) When a player sees a gap in league, he goes for it - no matter what. In union, players have to be sure that they are not running away from their support players, and therefore this restricts what a player can do. This is especially true on the rare occasions that aplayer runs the ball out of his own 22-meter area.The player must always be cautious to stay with supports, or not turn the ball over - once again, this greatly restricts the running options that a player has. Compare this to league, where the kick-return is often the greatest opportunity that a player has to stride out, and run at a staggered defensive line - this, in complete contrast to union, gives a player the chance to use their great evasive skills, and individuality, to maximum effect.
b) When a league player hits the line, he can try and go straight through, or can struggle and try and break the tackle. This is a much riskier proposition in union, as it is very easy to lose the ball. As a result, when a union player hits the line, they must do so with a safety first approach. They must ensure that they are able to fall correctly, when tackled, to ensure that they maintain possession - this restricts the creativity of players. And, rather than struggling through when they feel that there is a chance to break a tackle, they are, more than likely, inclined to turn and look for support, or, perhaps, start a maul.
Based on these reasons, I have no doubt at all that league players would not improve their running in union. In addition to this, the reduced amount of tackling would, no doubt, do little to improve their defensive games.