The biggest factor in golf equipment is not the heads, but the shafts. So while the heads change, most pro golfers use the same shafts their previous clubs, usually True Temper steel in the irons. Irons are the precision clubs and pro golfers dial in their distance with them. Drivers are constantly changed, even week to week, as the pros do get money for using them. But these are power and not precision instruments. Again, you can use the same shaft in the driver heads, but pros are more willing to try different driver shafts.
As noted, putters and wedges are usually unchanged and often different makes from their main sponsor (it's allowed in the contract). Many wedges are customized by grinding the sole and bending to get a stronger/weaker loft.