You can opt out of HP Ad program and can join again any time. No issues with this.
With respect to earning impact only you should be in a position to analyse the same. If you are already earning through Adsense and want to maximize it I guess you can try out opting out HP-Ad.
On the other hand if you have substantial HP-Ad earnings you tend to loose that. This is a call you only have to take.
In general, if your hubs are well SEO optimized and if your majority visitors are organic I guess you should go with Adsense only. If your hub traffic is more of referrals and have more hubpages member traffic then may be you should continue with HP Ad.
In short, worth taking the risk, so go ahead.