You may be able to replace should with if, but that is not the context of the passage from the original article that you sourced.
Before the author wrote those lines, he spoke to many other people who all stated that they expected the Reserve Bank to cut the rate. One actually stated that it was not a matter of if, but when. Their unanimous conclusion was a change in the value of the Australian dollar when this event occurred.
If you look at the verb definitions for should, you will find one that expresses a conditional mode (if). This is also like one of the verb definitions of should. However in both definitions the condition is specified in the sentence. For example:
"If it rains, you should take an umbrella with you."
Another definition of should indicates an event that is possible. Remember in the original article, all the people interviewed clearly expressed an expectation that the rate cut would happen and the consequences of that rate cut. So in this context, should can be replaced with when if the author's original intent is to be preserved in the standalone excerpt:
" OzForex Foreign Exchange dealer Michael Judge said the dollar could go above US79¢ on Tuesday when the Reserve Bank cuts the cash rate, which is currently at 2.25 per cent."
Therefore, although you could use if as a synonym in the excerpt, this will only denote a condition. However, if you want to denote the result of an expected event- as outlined by the author in the article- then you may want to use when as a replacement.