You should speak to your tax credits office before you decide whether to participate in a Salary Exchange Scheme. You must also notify your tax credits office once you have exchanged your salary.
However, in broad terms, as your gross salary reduces (and employer pension contributions are disregarded) your entitlement to tax credits may increase. If you currently make personal contributions to a pension scheme, then you are currently entitled to deduct the gross amount of the pension contribution from your earnings to calculate your tax credits. In this situation, therefore, there should be little or no change to your tax credits entitlement.