Can you do it? Yes
Is it worth it in my opinion? Not really.
Remember that it's the scaled score that affects your ATAR. Let's say you only put in the same amount of effort to specialist as you would for Further, and that means you only get a 25 study score. This would scale to a 35 - a scaled score of 35 in Further would require you to get nearly a 40 raw score.
So which seems more achievable to you - a raw score of 25 in spec, or a 39 in Further? Remembering that Further is highly competitive and requires you to be scoring higher than you'd need to in spec to get a particular study score.
In most cases, the answer is to do what you enjoy more and feel most capable of doing, and to ignore scaling. IMO, this is the one time you should pay attention to scaling - if you have the ability to do spec, even if only a little, you will almost assuredly score for higher in spec after scaling than you would in Further after scaling