Since you mention Articles 33(2) and (3) PCT, I assume that you requested International Preliminary Examination, and that the Examiner who drafted the International Preliminary Examination Report (IPRP) found that some of your claims are not novel.
However, the IPRP is just that - preliminary. You may respond to the IPRP to try and overcome the objections it raises (Article 34(2)(d) PCT), but this is not compulsory. In other words, you are not obliged to amend your application during the international (PCT) phase.
When you enter the national/regional phases with your PCT application, you will have at least one opportunity to amend your application (Article 41(1) PCT) within a certain time limit (which depends on the patent office).
Which brings me to my second point. Just because the IPRP says that some claims are unpatentable does not mean:
- that they really are (the Examiner might have misread one of the pieces of prior art, or he might have interpreted the prior art too broadly), or
- that they are in all jurisdictions (for example, there are not-insignificant differences in practice between the USPTO and the EPO concerning obviousness or chemical inventions).
Also, there is always the chance that each of the patent offices where you entered the national/regional phase will start examination from scratch, possibly starting from different prior art.
In this context, I usually find amending the PCT application during the international phase to be of marginal interest, at best.
I usually consider making amendments only after the application it has entered the national/regional phases and the national/regional Examiner has rejected some of the claims. (Which I may address by arguing against the rejections, or by amending the claims, depending on the situation.)