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?