Are you on open label now and actually taking the study drug or are you, as your lung function declines (even if just a little) potentially still on the placebo?
I do think that the integrity of the trial process would be better preserved by trying not to resume a treatment you are pretty certain will change your health- since you will know any improvement won't be due to the study drug (assuming you are on the study drug and not a placebo.) However, while that is beneficial to the integrity of the trial process and overall accuracy of the results, it's not necessarily going to be good for YOU. How much longer is the trial going to last? It would be unfortunate to see you hold off on a treatment that could help you and have it turn out you've spent all this time on a placebo, but if you were on open label... I don't know. This is a hard one, weighing the individual needs of the patient against the way the whole clinical trials process works. I wish you the best in your decision.