the general guidelines is that each H1 must follow the location&priviling wages shown in the LCA of the petition. If the location of work is changed, the new location must be within the mormal commutable distance(there is no single digit number of how many miles etc..)
So, if you change the work location(new client location), it must be within the commutable distance from the existing LCA locations(each LCA can mention max 3 work locations). If its not within the commutable distance, one must file a new LCA(no need of H1 amendment as such, just obtain the new LCA and attach to the existing H1).
About the client change (for the matter of client name), its Okay to change the client on H1, even if its not stamped. All you need to do is to take the current client letter (and also its safe to take the client letter at the time of H1 approval) for the current stamping. Dont show any of these client letters, unless and otherwise the officer asks at the time of interview for stamping.