For what it's worth - Suzuki offered two second generation four cylinder vehicles - the SQ416 & the SQ420 - the 1.6L G16B engined SQ416 has a smaller front brake assembly than the 2.0L J20A engined SQ420 - I just eyeballed my SQ420 (a '98GV with the optional factory 16x7 Enkei alloys) and the clearance to the rim is around 3/8 of an inch - the only way to get a 15" rim over those brakes will be to space it outwards, which is not something I would be comfortable doing.
The deciding factor here is going to be what brakes are on the vehicle the OP has, which by the way is a Vitara, not a Tracker - so I suggest you do what I just did - go out to the vehicle and take a look - you're looking specifically at the clearance between the caliper and the rim - if it's less than half an inch, with the 16" rims you now have, you will be unable to fit 15" rims.
A point worthy of note - in some cases the cross sectional profile of the rim will dictate whether or not it clears - I have another Suzuki on which I have upgraded the front brakes (going from a solid rotor to a larger ventilated one), the stock 13" steel rims will no longer clear the calipers, but the 13" alloys that I've been using do.