COPD exacerbation: This is the most likely diagnosis given the context, as patients with COPD are prone to exacerbations which can present with increased shortness of breath, cough, and sputum production, similar to pneumonia. The history of COPD and the presence of typical exacerbation symptoms would point towards this diagnosis.

Community-acquired pneumonia (CAP): This is a common and significant consideration in the differential diagnosis, especially if the patient presents with fever, cough, sputum production, and shortness of breath. The clinical presentation can overlap significantly with COPD exacerbation.
Acute bronchitis: Although often considered a less severe condition, acute bronchitis can present with cough and sputum production, potentially mimicking a COPD exacerbation or early pneumonia.

Pulmonary embolism: This is a critical diagnosis not to miss, as it can present with sudden onset of shortness of breath and can be fatal if not promptly treated. Patients with COPD are at increased risk of pulmonary embolism.
Cardiac ischemia or heart failure: These conditions can also present with shortness of breath and may be precipitated by or mimic a COPD exacerbation or pneumonia, especially in patients with pre-existing heart disease.
Tuberculosis (TB): Although less common in some regions, TB can present with chronic cough, sputum production, and shortness of breath, overlapping with symptoms of COPD exacerbation and pneumonia.

Cystic fibrosis exacerbation (in patients with known cystic fibrosis): This would be more relevant in younger patients or those with a known history of cystic fibrosis.
Bronchiectasis: This condition involves permanent dilation of parts of the airways and can lead to chronic cough and sputum production, potentially mimicking a COPD exacerbation.
Interstitial lung disease (ILD) exacerbation: Patients with underlying ILD can experience exacerbations that might present with worsening shortness of breath and could be confused with COPD exacerbation or pneumonia.