Looking at the current
2009-2010 BCAPL rules, I do not see a 1.4.c section at all. Maybe 1.4.c is from an older set of rules.
Under section 1.43.1 it is a concession of the game, but not the match, if you put your playing cue away. This does seem rather odd as I cannot fathom why anyone would put away their cue for a single game in a match. However, it does not say it is a concession for just unscrewing it. It seems to me that if you unscrew your playing cue to change shafts then you have not put your cue away and therefore is not a concession. On the other hand, 1.43.1 also leaves it open to the interpretation of the referee as to whether or not any of your actions indicated a concession.
Under section 1.44 it is clear that unscrewing your playing cue while your opponent is on the hill AND it is during your opponents inning, it is considered a concession. This appears identical to WPA rule 1.11. I did find it interesting that 1.44 only applies to the playing cue, not the break cue. This is from page 96 in the Applied Rulings section:
1. Situation: Players A and B are both on the hill. Player A, having broken the final game of the match with a break cue and not pocketing a ball, retires to the chair. While Player B is at the table, Player A, having their playing cue standing by, unscrews their break cue to put it away.
Ruling: No concession. The cue Player A unscrewed was not their playing cue.
It is also worth noting that it can still be judged as unsportsmanlike conduct under 1.45. This is also from page 96 in the Applied Rulings section.
While it is impossible to conceive every potential example, the situation from the above discussion of Rule 1.44, Concession of Match, will serve to demonstrate the principle. Despite the specific wording of Rule 1.44 and the Applied Ruling that goes with it, if a referee judged that the timing, body language, relative positions of the players, or any other pertinent information relative to the unscrewing of the break cue gave the clear overall effect of an unsportsmanlike act, the player would still be subject to penalties under Rule 1.45.
All in all, it appears to be legal to change shafts during a game. I would however consider it proper etiquette to only perform such actions during your turn and not your opponents. I would also consider it proper to inform your opponent that you are changing shafts before doing so. This would alleviate any confusion and allow an opportunity to involve a referee in the case that there is a difference of opinions regarding the rules.
To answer the OP, it appears to be legal to put away your break cue before the end of the match as long as it cannot be construed that you were trying to shark your opponent. Personally I would prefer to just keep all of my equipment assembled until the end of the match. That way there would be no arguments.
Of course, all of the above regards current BCAPL rules only. Other leagues might vary in how they rule on these situations.
John