It really depends on what they do when they implement it in WCF.
.Net web services are by definition supposed to be platform and implementation agnostic. The client shouldn't need to care what the service was written in or visa versa. For the most part, when services are done correctly, it works that way.
The problem though is that web services standards are also being updated and new ones introduced over time. The ASP.Net web service technology that is used in PowerBuilder Classic is a bit dated, and there are a number of things that they might end up doing in the WCF service that might make it hard for an ASP.Net based web service client to consume it.
The most likely issue would be that they would use the DataContractSerializer (the default one for WCF) when creating the service. The ASP.Net client will attempt to use a XMLSerializer. The WCF service can be configured to use the XMLSerializer instead.