Is it worthwhile to upgrade an old but quality DAC?

I personally don’t think so. DAC designs have come so far in the last few years, particularly in how they handle jitter, and they’re much more flexible and future proof now with networking capabilities and format handling.

I believe that Altis is almost 20 years old - I wouldn’t be surprised if a budget DAC from Schiit or iFi would sound better than it, let alone a modern high-end DAC.

would agree with @hudsonhawk , even a Topping D10 or D30 are going to outperform your old DAC, have XMOS chip for USB input have the later chip set and will cost you less in the long run.

Agree.  Moses, come to the promised land of modern DACs

Keep what you have now.  Higher sample and bit rates are good, but the primary advances in recent D/A technology have been in making 44/16 recordings sound their best.  From the description of your setup I assume you think the Altis sounds better than the Oppo at 44/16.  Does the Altis also sound better than the Oppo when the Oppo is using higher sample/bit rates?

Thank you and appreciate all for your advice. When playing regular CD using Oppo 105 which connects directly to my Doshi preamp, or connect to Altis Reference which then  connects to Doshi Preamp, the quality of sound seems slightly better with Altis connection. When playing SACD, Altis Reference will not transmit the sound  as it only works with 44/16.  Thus I agree it is not worthwhile to upgrade Altis per your advice . 
While on this issue and being naive on digital/streaming, any suggestion from Audiogon experts if sound quality of Oppo 105 can be improved with streaming Tidal Master level music files. In comparison when I connect CEC TL0 to Oppo 105 Coaxial input, the SQ of playing CD is improved quite noticeably. Thus,  I wonder what would be the practical way to improve SQ when streaming with Oppo 105 for the  high resolution source such as Tidal Master level music file? 

I would not say my 105 is reference quality.  My emotiva pre sounds better and my Chord crushes it.  The 105 is great for SACD and theater.

Time for something newer. 
Most good modern dacs will sound heaps better than the 105, which I owned happily for a number of years. If the Altis only modestly beats it on redbook, you've got plenty of opportunity to get something that will be a large enhancement. Cheers,
Spencer

You could add a reclocker in front of the dac. This might improve the sound. It won't give you 24/96 support. 

It is basically impossible to upgrade an old dac to high bit rate unless the manufacturer says they can do it. Some expensive dacs has the dac chip on a small board the could  be swapped. 

Might be better to buy a new dac.